NomenclatureMany United States Army installations are known as ''forts'', although they are not always fortified. Indeed, during the pioneering era of North America, many outposts on the frontiers, even non-military outposts, were referred to generically as ''forts''. Larger military installations may be called ''fortresses''; smaller ones were once known as ''fortalices''. The word ''fortification'' can also refer to the practice of improving an area's defense with defensive works. City are fortifications but are not necessarily called ''fortresses''. The art of setting out a military camp or constructing a fortification traditionally has been called '' metation'' since the time of the s. The art/science of laying to a fortification and of destroying it is commonly called '' '' or ''siege warfare'' and is formally known as '' ''. In some texts this latter term also applies to the art of building a fortification. Fortification is usually divided into two branches: permanent fortification and field fortification. Permanent fortifications are erected at leisure, with all the resources that a state can supply of constructive and mechanical skill, and are built of enduring materials. Field fortifications—for example breastworks—and often known as ''fieldworks'' or ''earthworks'', are extemporized by troops in the field, perhaps assisted by such local labour and tools as may be procurable and with materials that do not require much preparation, such as , brushwood and light , or sandbags (see sangar). An example of field fortification was the construction of by George Washington in 1754. There is also an intermediate branch known as ''semi-permanent fortification''. This is employed when in the course of a campaign it becomes desirable to protect some locality with the best imitation of permanent defences that can be made in a short time, ample resources and skilled civilian labour being available. An example of this is the construction of Roman forts in England and in other Roman territories where camps were set up with the intention of staying for some time, but not permanently. s are fortifications which are regarded as being distinct from the generic fort or fortress in that it describes a residence of a or and commands a specific defensive territory. An example of this is the massive medieval castle of .
AncientFrom very early history to modern times, have been a necessity for many cities. In , near the town of a walled fortified settlement today called starting from 4700 BC had a diameter of about 300 feet (100 meters), was home to 350 people living in two-storey houses, and was encircled by a fortified wall. The huge walls around the settlement, which were built very tall and with stone blocks which are 6 feet (2 meters) high and 4.5 feet (1.5 meters) thick, make it one of the earliest walled settlements in Europe but it is younger than the walled town of in [[Greece from 6800 BC. [[Uruk in ancient [[Sumer ([[Mesopotamia) is one of the world's oldest known [[walled cities. The Ancient Egyptians also built fortresses on the frontiers of the [[Nile Valley to protect against invaders from neighbouring territories, as well as circle-shaped mud brick walls around their cities. Many of the fortifications of the ancient world were built with mud brick, often leaving them no more than mounds of dirt for today's archaeologists. A massive prehistoric stone wall surrounded the ancient temple of [[Ness of Brodgar 3200 BC in [[Scotland. Named the "Great Wall of Brodgar" it was four metres thick and four metres tall. The wall had some symbolic or ritualistic function. The [[ancient Assyrians|Assyrians deployed large labour forces to build new [[palaces, temples and [[defensive walls. Some settlements in the were also fortified. By about 3500 BC, hundreds of small farming villages dotted the [[Indus floodplain. Many of these settlements had fortifications and planned streets. The stone and mud brick houses of [[Kot Diji were clustered behind massive stone flood dykes and defensive walls, for neighbouring communities bickered constantly about the control of prime agricultural land. [[Mundigak (c. 2500 BC) in present-day south-east [[Afghanistan has defensive walls and square [[bastions of sun dried bricks. The entire city of [[Kerma in [[Nubia was encompassed by fortified walls surrounded by a ditch. Archaeology has revealed various Bronze age bastions and foundations constructed of stone together with either baked or unfired brick. In [[Bronze Age [[Malta, some settlements also began to be fortified. The most notable surviving example is [[Borġ in-Nadur, where a bastion built in around 1500 BC was found. [[Babylon was one of the most famous cities of the ancient world, especially as a result of the building program of [[Nebuchadnezzar, who expanded the walls and built the [[Ishtar Gate. Exceptions were few—notably, ancient [[Sparta and ancient [[Rome did not have walls for a long time, choosing to rely on their militaries for defence instead. Initially, these fortifications were simple constructions of wood and earth, which were later replaced by mixed constructions of stones piled on top of each other without [[Mortar (masonry)|mortar. In , large stone walls had been built in , such as the ancient site of (famous for the huge stone blocks of its ' ' walls). In [[classical era [[Greece, the city of [[Athens built two parallel stone walls, called the [[Long Walls, that reached their fortified seaport at [[Piraeus a few miles away. In [[Central Europe, the [[Celts built large fortified settlements known as [[oppidum|oppida, whose walls seem partially influenced by those built in the [[Mediterranean. The fortifications were continuously being expanded and improved. Around 600 BC, in [[Heuneburg, Germany, forts were constructed with a limestone foundation supported by a [[mudbrick wall approximately 4 metres tall, probably topped by a roofed walkway, thus reaching a total height of 6 metres. The wall was clad with lime plaster, regularly renewed. Towers protruded outwards from it. The [[Oppidum of Manching (German: Oppidum von Manching) was a large Celtic proto-urban or city-like settlement at modern-day Manching (near Ingolstadt), Bavaria (Germany). The settlement was founded in the 3rd century BC and existed until c. 50–30 BC. It reached its largest extent during the late La Tène period (late 2nd century BC), when it had a size of 380 hectares. At that time, 5,000 to 10,000 people lived within its 7.2 km long walls. The oppidum of [[Bibracte is another example of a Gaulish fortified settlement. The [[Mura aureliane are a line of [[city walls built between 271 AD and 275 AD in [[Rome, [[Italy, during the reign of the [[Roman Emperors [[Aurelian and [[Marcus Aurelius Probus|Probus. The walls enclosed all the [[seven hills of Rome plus the [[Campus Martius and, on the right bank of the [[Tiber, the [[Trastevere district. The river banks within the city limits appear to have been left unfortified, although they were fortified along the Campus Martius. The full circuit ran for surrounding an area of . The walls were constructed in brick-faced concrete, thick and high, with a square tower every 100 Roman feet (). In the 5th century, remodelling doubled the height of the walls to . By 500 AD, the circuit possessed 383 towers, 7,020 [[crenellations, 18 main gates, 5 [[postern|postern gates, 116 [[latrines, and 2,066 large external windows.Claridge, Amanda (1998). ''Rome: An Oxford Archaeological Guide'', First, Oxford, UK: Oxford University Press, 1998, pp. 59, 332–335. The [[ancient Rome|Romans fortified their cities with massive, mortar-bound stone walls. The most famous of these are the largely extant [[Aurelian Walls of [[Rome and the [[Theodosian Walls of [[Constantinople, together with partial remains elsewhere. These are mostly city gates, like the [[Porta Nigra in [[Trier or [[Newport Arch in [[Lincoln, Lincolnshire|Lincoln. [[Hadrian's Wall was built by the [[Roman Empire across the width of what is now [[northern England following a visit by [[List of Roman Emperors|Roman Emperor [[Hadrian (AD 76–138) in AD 122.
IndiaA number of forts dating from the [[Later Stone Age to the [[British Raj may be found in India. "Fort" is the word used in India for all old fortifications. Numerous [[Indus Valley Civilisation|Indus Valley Civilization sites exhibit evidences of fortifications. While [[Dholavira has stone-built fortification walls, [[Harappa|Harrapa is fortified using baked bricks; sites such as [[Kalibangan exhibit [[mudbrick fortifications with bastions and [[Lothal has a quadrangular fortified layout. Evidence also suggested of fortifications in [[Mohenjo-daro. Even a small town – for instance, Kotada Bhadli, exhibiting sophisticated fortification-like bastions – shows that nearly all major and minor towns of the Indus Valley Civilization were fortified. Forts also appeared in urban cities of the Gangetic valley during the second urbanisation period between 600–200 BC, and as many as 15 fortification sites have been identified by archaeologists throughout the Gangetic valley, such as [[Kosambi|Kaushambi, [[Mahasthangarh, [[Pataliputra, [[Mathura, [[Ahichchhatra, [[Rajgir, and [[Lauria Nandangarh. The earliest vedic brick fortification occurs in one of the stupa mounds of Lauria Nandangarh, which is 1.6 km in perimeter and oval in plan and encloses a habitation area. India currently has over 180 forts, with the state of [[Maharashtra alone having over 70 forts, which are also known as ''durg'', many of them built by [[Shivaji, founder of the [[Maratha Empire|Maratha state. A large majority of forts in India are in North India. The most notable forts are the [[Red Fort at [[Delhi, the [[Agra Fort|Red Fort at [[Agra, the [[Chittor Fort and [[Mehrangarh Fort in [[Rajasthan, the [[Ranthambhor Fort, [[Amer Fort and [[Jaisalmer Fort also in Rajasthan and [[Gwalior Fort in [[Madhya Pradesh.
ChinaLarge tempered earth (i.e. [[rammed earth) walls were built in [[ancient China since the [[Shang dynasty (c. 1600–1050 BC); the capital at ancient Ao had enormous walls built in this fashion (see for more info). Although stone walls were built in China during the [[Warring States (481–221 BC), mass conversion to stone architecture did not begin in earnest until the [[Tang dynasty (618–907 AD). The [[Great Wall of China had been built since the [[Qin dynasty (221–207 BC), although its present form was mostly an engineering feat and remodelling of the [[Ming dynasty (1368–1644 AD). In addition to the Great Wall, a number of Chinese cities also employed the use of s to defend their cities. Notable [[Chinese city walls include the city walls of [[Hangzhou City Walls|Hangzhou, [[City Wall of Nanjing|Nanjing, the [[Old City (Shanghai)|Old City of Shanghai, [[City Wall of Suzhou|Suzhou, [[Fortifications of Xi'an|Xi'an and the [[walled villages of Hong Kong. The famous walls of the [[Forbidden City in [[Beijing were established in the early 15th century by the [[Yongle Emperor. The Forbidden City made up the inner portion of the [[Beijing city fortifications.
Spanish colonial fortificationsDuring the [[Spanish East Indies|Spanish Era several forts and outposts were built throughout the archipelago. Most notable is [[Intramuros, the old [[defensive walls|walled city of Manila located along the southern bank of the [[Pasig River. The historic city was home to centuries-old churches, schools, convents, government buildings and residences, the best collection of Spanish colonial architecture before much of it was destroyed by the bombs of [[World War II. Of all the buildings within the 67-acre city, only one building, the San Agustin Church, survived the war. Partial listing of Spanish forts: # [[Intramuros, [[Manila # [[Cuartel de Santo Domingo, [[Santa Rosa, Laguna # [[Cuyo Fort|Fuerza de Cuyo, [[Cuyo, Palawan # [[Cagayancillo Fort|Fuerza de Cagayancillo, [[Cagayancillo, [[Palawan # [[Fort Pilar|Real Fuerza de Nuestra Señora del Pilar de Zaragoza, [[Zamboanga City # [[Fort San Felipe (Cavite)|Fuerza de San Felipe, [[Cavite City # [[Fort San Pedro|Fuerza de San Pedro, [[Cebu # [[Fuerte dela Concepcion y del Triunfo, [[Ozamiz, [[Misamis Occidental # [[Fort San Antonio Abad|Fuerza de San Antonio Abad, [[Manila # [[Fort Pikit|Fuerza de Pikit, [[Pikit, Cotabato # [[Romblon, Romblon|Fuerza de Santiago, [[Romblon, Romblon # [[Jolo, Sulu|Fuerza de Jolo, [[Jolo, Sulu # [[Masbate|Fuerza de Masbate, [[Masbate # [[Bongabong, Oriental Mindoro|Fuerza de Bongabong, [[Bongabong, Oriental Mindoro # [[Dapitan|Cotta de Dapitan, [[Dapitan, [[Zamboanga del Norte # [[Tukuran, Zamboanga del Sur|Fuerte de Alfonso XII, [[Tukuran, Zamboanga del Sur # [[Bacolod, Lanao del Norte|Fuerza de Bacolod, [[Bacolod, Lanao del Norte # [[Guinsiliban, Camiguin|Guinsiliban Watchtower, [[Guinsiliban, Camiguin # [[Laguindingan, Misamis Oriental|Laguindingan Watchtower, [[Laguindingan, Misamis Oriental # [[Gumaca|Kutang San Diego, [[Gumaca, Quezon # [[Luna, La Union|Baluarte Luna, [[Luna, La Union
Local fortificationsThe Ivatan people of the northern islands of Batanes built their so-called ''[[idjang'' on hills and elevated areas to protect themselves during times of war. These fortifications were likened to European castles because of their purpose. Usually, the only entrance to the castles would be via a rope ladder that would only be lowered for the villagers and could be kept away when invaders arrived. The [[Igorot people|Igorots built forts made of stone walls that averaged several meters in width and about two to three times the width in height around 2000 BC.Ancient and Pre-Spanish Era of the Philippines
IslamicDuring Muhammad's era in Arabia, many tribes made use of fortifications. In the [[Battle of the Trench, the largely outnumbered defenders of Medina, mainly [[Muslims led by [[Prophets of Islam|Islamic prophet [[Muhammad, dug a trench, which together with Medina's natural fortifications, rendered the confederate [[cavalry (consisting of horses and [[Camel cavalry|camels) useless, locking the two sides in a stalemate. Hoping to make several attacks at once, the confederates persuaded the Medina-allied [[Banu Qurayza to attack the city from the south. However, [[Muhammad as a diplomat|Muhammad's diplomacy derailed the negotiations, and broke up the confederacy against him. The well-organized defenders, the sinking of confederate morale, and poor weather conditions caused the siege to end in a fiasco.* During the [[Siege of Ta'if in January 630, Note: Shawwal 8AH is January 630AD Muhammad ordered his followers to attack enemies who fled from the [[Battle of Hunayn and sought refuge in the fortress of Taif.William Muir, The life of Mahomet and history of Islam to the era of the Hegira, Volume 4, p. 142.
AfricaThe [[walls of Benin are described as the world's second longest man-made structure, as well as the most extensive earthwork in the world, by the Guinness Book of Records, 1974. The walls may have been constructed between the thirteenth and mid-fifteenth century CE or, during the first millennium CE. Strong citadels were also built other in areas of Africa. [[History of the Yoruba people|Yorubaland for example had several sites surrounded by the full range of earthworks and ramparts seen elsewhere, and sited on ground. This improved defensive potential- such as hills and ridges. Yoruba fortifications were often protected with a double wall of trenches and ramparts, and in the Congo forests concealed [[ditches and paths, along with the main works, often bristled with rows of sharpened stakes. Inner defenses were laid out to blunt an enemy penetration with a maze of defensive walls allowing for entrapment and [[crossfire on opposing forces.July, pp. 11–39 A military tactic of the [[Ashanti Empire|Ashanti was to create powerful log [[stockades at key points. This was employed in later wars against the [[British Empire|British to block British advances. Some of these fortifications were over a hundred yard long, with heavy parallel tree trunks. They were impervious to destruction by artillery fire. Behind these stockades numerous Ashanti soldiers were mobilized to check enemy movement. While formidable in construction, many of these strongpoints failed because Ashanti guns, gunpowder and bullets were poor, and provided little sustained killing power in defense. Time and time again British troops overcame or bypassed the stockades by mounting old-fashioned bayonet charges, after laying down some covering fire. Defensive works were of importance in the tropical African Kingdoms. In the [[Kingdom of Kongo field fortifications were charactarized by trenches and low earthen embankments. Such strongpoints ironically, sometimes held up much better against European cannon than taller, more imposing structures.Thornton, pp. 22–39
Medieval Europeand s were the main antecedents of [[castles in , which emerged in the 9th century in the . The saw the creation of some towns built around castles. These cities were only rarely protected by simple stone walls and more usually by a combination of both walls and . From the 12th century hundreds of settlements of all sizes were founded all across Europe, which very often obtained the right of fortification soon afterwards. The founding of urban centres was an important means of territorial expansion and many cities, especially in [[eastern Europe, were founded precisely for this purpose during the period of [[Ostsiedlung|Eastern Colonisation. These cities are easy to recognise due to their regular layout and large market spaces. The fortifications of these settlements were continuously improved to reflect the current level of military development. During the [[Renaissance era, the [[Republic of Venice|Venetian Republic raised great walls around cities, and the finest examples, among others, are in [[Nicosia (Cyprus), [[Rocca di Manerba del Garda (Lombardy) and [[Palmanova (Italy), or [[Dubrovnik (Croatia), which proved to be futile against attacks but still stand to this day. Unlike Venetians the [[Ottoman architecture|Ottomans used to built smaller fortifications but in greater numbers, and only rarely fortified entire settlements such as [[Počitelj, [[Walled city of Vratnik|Vratnik and [[Walled city of Jajce|Jajce in [[Ottoman Bosnia|Bosnia.
Development after introduction of firearmsMedieval-style fortifications were largely made obsolete by the arrival of s on the 14th century [[battlefield. Fortifications in the age of evolved into much lower structures with greater use of and [[earthworks (engineering)|earth ramparts that would absorb and disperse the energy of cannon fire. Walls exposed to direct cannon fire were very vulnerable, so were sunk into ditches fronted by earth slopes. This placed a heavy emphasis on the [[geometry of the fortification to allow defensive cannonry interlocking fields of fire to cover all approaches to the lower and thus more vulnerable walls. The evolution of this new style of fortification can be seen in transitional forts such as Sarzanello in North West Italy which was built between 1492 and 1502. Sarzanello consists of both crenellated walls with towers typical of the medieval period but also has a [[ravelin like angular gun platform screening one of the curtain walls which is protected from flanking fire from the towers of the main part of the fort. Another example are the [[fortifications of Rhodes which were ''frozen'' at 1522 so that Rhodes is the only European walled town that still shows the transition between the classical medieval fortification and the modern ones. Fortifications also extended in depth, with protected batteries for defensive cannonry, to allow them to engage attacking cannon to keep them at a distance and prevent them bearing directly on the vulnerable walls. The result was [[Star fort|star shaped fortifications with tier upon tier of hornworks and [[bastions, of which [[Fort Bourtange is an excellent example. There are also extensive fortifications from this era in the [[Northern Europe|Nordic states and in [[Great Britain|Britain, the fortifications of [[Berwick-upon-Tweed and the harbour [[archipelago of [[Suomenlinna at [[Helsinki being fine examples.
19th centuryThe arrival of s in the 19th century led to yet another stage in the evolution of fortification. [[Star forts did not fare well against the effects of high explosive and the intricate arrangements of bastions, flanking batteries and the carefully constructed lines of fire for the defending cannon could be rapidly disrupted by explosive shells. Worse, the large open ditches surrounding forts of this type were an integral part of the defensive scheme, as was the covered way at the edge of the [[counter scarp. The ditch was extremely vulnerable to bombardment with explosive shells. In response, military engineers evolved the [[Polygonal fort|polygonal style of fortification. The ditch became deep and vertically sided, cut directly into the native rock or soil, laid out as a series of straight lines creating the central fortified area that gives this style of fortification its name. Wide enough to be an impassable barrier for attacking troops, but narrow enough to be a difficult target for enemy shellfire, the ditch was swept by fire from defensive [[blockhouses set in the ditch as well as firing positions cut into the outer face of the ditch itself. The profile of the fort became very low indeed, surrounded outside the ditch covered by [[caponiers by a gently sloping open area so as to eliminate possible cover for enemy forces, while the fort itself provided a minimal target for enemy fire. The entrypoint became a sunken gatehouse in the inner face of the ditch, reached by a curving ramp that gave access to the gate via a rolling bridge that could be withdrawn into the gatehouse. Much of the fort moved underground. Deep passages and [[tunnels now connected the [[blockhouses and firing points in the ditch to the fort proper, with [[magazine (artillery)|magazines and machine rooms deep under the surface. The guns, however, were often mounted in open emplacements and protected only by a [[parapet; both in order to keep a lower profile and also because experience with guns in closed [[casemates had seen them put out of action by rubble as their own casemates were collapsed around them. Gone were citadels surrounding towns: forts were to be moved to the outside of the cities some 12 km to keep the enemy at a distance so their artillery could not bombard the city center. From now on a ring of forts were to be built at a spacing that would allow them to effectively cover the intervals between them. The new forts abandoned the principle of the bastion, which had also been made obsolete by advances in arms. The outline was a much simplified polygon, surrounded by a ditch. These forts, built in masonry and shaped stone, were designed to shelter their garrison against bombardment. One organizing feature of the new system involved the construction of two defensive curtains: an outer line of forts, backed by an inner ring or line at critical points of terrain or junctions (see, for example, [[Séré de Rivières system in France). Traditional fortification however continued to be applied by European armies engaged in warfare in colonies established in Africa against lightly armed attackers from amongst the indigenous population. A relatively small number of defenders in a fort impervious to primitive weaponry could hold out against high odds, the only constraint being the supply of ammunition.
20th and 21st centuries[[Steel-and-[[concrete fortifications were common during the 19th and early 20th centuries. However the advances in modern warfare since [[World War I have made large-scale fortifications [[obsolete in most situations. In the 1930s and 1940s, some fortifications were built with designs taking into consideration the new threat of [[aerial warfare, for example [[Fort Campbell (Malta)|Fort Campbell in Malta. Despite this, only underground [[bunkers are still able to provide some protection in modern wars. Many historical fortifications were demolished during the modern age, but a considerable number survive as popular tourist destinations and prominent local [[landmarks today. The downfall of permanent fortifications had two causes: * The ever-escalating power, speed, and reach of artillery and air power meant that almost any target that could be located could be destroyed, if sufficient force were massed against it. As such, the more resources a defender devoted to reinforcing a fortification, the more combat power that fortification justified being devoted to destroying it, if the fortification's destruction was demanded by an attacker's strategy. From [[World War II, [[bunker busters were used against fortifications. By 1950, [[nuclear weapons were capable of destroying entire cities, and produced dangerous [[radiation. This led to the creation of civilian nuclear [[air raid shelters. * The second weakness of permanent fortification was its very permanency. Because of this it was often easier to go around a fortification and, with the rise of mobile warfare in the beginning of World War II, this became a viable offensive choice. When a defensive line was too extensive to be entirely bypassed, massive offensive might could be massed against one part of the line allowing a breakthrough, after which the rest of the line could be bypassed. Such was the fate of the many defensive lines built before and during World War II, such as the [[Siegfried Line, the [[Stalin Line and the [[Atlantic Wall. This was not the case with the [[Maginot Line; it was designed to force the Germans to invade other countries (Belgium or Switzerland) to go around it, and was successful in that sense.[Halter, Marc; History of the Maginot Line, Moselle River, 2011. ] Instead field fortification rose to dominate defensive action. Unlike the [[trench warfare which dominated [[World War I, these defences were more temporary in nature. This was an advantage because since it was less extensive it formed a less obvious target for enemy force to be directed against. If sufficient power were massed against one point to penetrate it, the forces based there could be withdrawn and the line could be re-established relatively quickly. Instead of a supposedly impenetrable defensive line, such fortifications emphasized [[defence in depth, so that as defenders were forced to pull back or were overrun, the lines of defenders behind them could take over the defence. Because the mobile offensives practised by both sides usually focused on avoiding the strongest points of a defensive line, these defences were usually relatively thin and spread along the length of a line. The defence was usually not equally strong throughout however. The strength of the defensive line in an area varied according to how rapidly an attacking force could progress in the terrain that was being defended—both the terrain the defensive line was built on and the ground behind it that an attacker might hope to break out into. This was both for reasons of the strategic value of the ground, and its defensive value. This was possible because while offensive tactics were focused on mobility, so were defensive tactics. The dug in defences consisted primarily of infantry and antitank guns. Defending tanks and tank destroyers would be concentrated in mobile [[brigades behind the defensive line. If a major offensive was launched against a point in the line, mobile reinforcements would be sent to reinforce that part of the line that was in danger of failing. Thus the defensive line could be relatively thin because the bulk of the fighting power of the defenders was not concentrated in the line itself but rather in the mobile reserves. A notable exception to this rule was seen in the defensive lines at the [[Battle of Kursk during [[World War II, where [[Germany|German forces deliberately attacked into the strongest part of the [[Soviet Union|Soviet defences seeking to crush them utterly. The [[terrain that was being defended was of primary importance because [[open terrain that tanks could move over quickly made possible rapid advances into the defenders' rear areas that were very dangerous to the defenders. Thus such terrain had to be defended at all cost. In addition, since in theory the defensive line only had to hold out long enough for mobile reserves to reinforce it, terrain that did not permit rapid advance could be held more weakly because the enemy's advance into it would be slower, giving the defenders more time to reinforce that point in the line. For example, the [[battle of the Hurtgen Forest in [[Germany during the closing stages of [[World War II is an excellent example of how difficult terrain could be used to the defenders' advantage. After World War II, [[ICBMs capable of reaching much of the way around the world were developed, and so speed became an essential characteristic of the strongest militaries and defenses. [[Missile silos were developed, so missiles could be fired from the middle of a country and hit cities and targets in another country, and airplanes (and air carriers) became major defenses and offensive weapons (leading to an expansion of the use of airports and airstrips as fortifications). Mobile defenses could be had underwater, too, in the form of [[nuclear submarines capable of firing missiles. Some bunkers in the mid to late 20th century came to be buried deep inside mountains and prominent rocks, such as [[Gibraltar and the [[Cheyenne Mountain Complex. On the ground itself, [[minefields have been used as hidden defences in modern warfare, often remaining long after the wars that produced them have ended. [[Demilitarized zones along borders are arguably another type of fortification, although a passive kind, providing a buffer between potentially hostile militaries.
Military airfieldsMilitary airfields offer a fixed "target rich" environment for even relatively small enemy forces, using [[hit-and-run tactics by ground forces, stand-off attacks (mortars and rockets), air attacks, or ballistic missiles. Key targets – aircraft, munitions, fuel, and vital technical personnel – can be protected by fortifications. Aircraft can be protected by [[revetments, [[Hesco barriers, or hardened aircraft shelters which will protect from many types of attack. Larger aircraft types tend to be based outside the operational theatre. Munition storage follows safety rules which use fortifications (bunkers and bunds) to provide protection against accident and chain reactions (sympathetic detonations). Weapons for rearming aircraft can be stored in small fortified ''expense'' stores closer to the aircraft. At Bien Hoa South Vietnam on the morning of 16 May 1965, as aircraft were being re-fuelled and armed, a chain reaction explosion destroyed 13 aircraft, killed 34 personnel, and injured over 100; this, along with damage and losses of aircraft to enemy attack (by both [[Infiltration tactics|infiltration and stand off attacks), led to the construction of revetments and shelters to protect aircraft throughout South Vietnam. Aircrew and ground personnel will need protection during enemy attacks and fortifications range from culvert section "duck and cover" shelters to permanent air-raid shelters. Soft locations with high personnel densities such as accommodation and messing facilities can have limited protection by placing prefabricated concrete walls or barriers around them, examples of barriers are Jersey Barriers, T Barriers or Splinter Protection Units (SPUs). Older fortification may prove useful such as the old 'Yugo' pyramid shelters built in the 1980s which were used by US personnel on 8 Jan 2020 when Iran fired 11 ballistic missiles at Ayn al-Asad Airbase in Iraq. Fuel is volatile and has to comply with rules for storage which provide protection against accident. Fuel in underground bulk fuel installations is well protected though valves and controls are vulnerable to enemy action. Above ground tanks can be susceptible to attack. Ground support equipment will need to be protected by fortifications to be useable after an enemy attack. Permanent (concrete) guard fortifications are safer, stronger, last longer and are more cost effective than sandbag fortifications. Prefabricated positions can be made from concrete culvert sections. The British Yarnold Bunker is made from sections of a concrete pipe. Guard Towers provide increased field of view but a lower level of protection. Dispersal and camouflage of assets can supplement fortifications against some forms of airfield attack.
Counter-insurgencyJust as in colonial periods, comparatively obsolete fortifications are still used for low-intensity conflicts. Such fortifications range in size from small patrol bases or [[forward operating bases up to huge [[airbases such as [[Camp Bastion/[[Camp Leatherneck|Leatherneck in [[Afghanistan. Much like in the 18th and 19th century, because the enemy is not a powerful military force with the heavy weaponry required to destroy fortifications, walls of [[gabion, [[sandbag or even simple mud can provide protection against small arms and anti-tank weapons – although such fortifications are still vulnerable to mortar and artillery fire.
FortsForts in modern American usage often refer to space set aside by governments for a permanent military facility; these often do not have any actual fortifications, and can have specializations (military barracks, administration, medical facilities, or intelligence). However, there are some modern fortifications that are referred to as forts. These are typically small semi permanent fortifications. In urban combat they are built by upgrading existing structures such as houses or public buildings. In field warfare they are often log, sandbag or [[gabion type construction. Such forts are typically only used in low level conflict, such as counterinsurgency conflicts or very low level conventional conflicts, such as the [[Indonesia–Malaysia confrontation, which saw the use of log forts for use by forward [[platoons and [[Company (military unit)|companies. The reason for this is that static above ground forts can not survive modern direct or indirect fire weapons larger than mortars, RPGs and small arms.
Prisons and othersFortifications designed to keep the inhabitants of a facility in rather than out can also be found, in [[prisons, [[concentration camps, and other such facilities, with [[supermaxes having some of the strongest of those. Those are covered in other articles, as most prisons and concentration camps are not primarily military forts (although forts, camps, and garrison towns have been used as prisons and/or concentration camps; such as [[Theresienstadt, [[Guantanamo Bay detention camp and the [[Tower of London for example).
See also* [[Border fence * [[Castra * [[Cavin * [[Citadel * [[Coastal fortification * [[Defense line * [[Defensive wall * [[Hesco bastion * [[Kuruwa, walls of a Japanese castle * [[List of fortifications * [[List of forts * [[Military camp * [[Slighting Fort components * [[Abatis * [[Banquette * [[Barbed wire, [[razor wire, [[wire entanglement, and [[wire obstacle * [[Bartizan * [[Bastion * [[Berm * [[Capital (fortification)|Capital * [[Caponier * [[Casemate * [[Castle walls * [[Czech hedgehog * [[Defensive fighting position * [[Ditch (fortification)|Ditch * [[Embrasure * [[Glacis * [[Gun turret * [[Keep * [[Lunette (fortification)|Lunette * [[Machicolation * [[Outwork * [[Palisade * [[Parapet#Parapets in fortification|Parapet * [[Pillbox (military)|Pillbox * [[Postern * [[Ravelin * [[Revetment * [[Sandbag * [[Sangar (fortification)|Sangar * [[Counterscarp|Scarp and Counterscarp * [[Turret * ''[[Zwinger'' Types of forts and fortification * [[Blockhouse * [[Bunker * * [[Chinese city wall * [[Compound (fortification)|Compound * [[Defensive wall * [[Kaiping Diaolou|Diaolou * [[Fire support base * [[Flak tower * [[Fortress church or [[fortified church * [[Gord (archaeology)|Grad, a Slavic wooden fortified settlement * [[Gusuku, fortifications in the Ryukyu Islands * [[Korean fortress * [[Hill fort * [[Land battery * [[Laneh Muri * [[Martello tower * [[Medieval fortification * [[Missile launch facility * [[Pā, a 19th-century Māori fortification * [[Peel tower * [[Polygonal fort * [[Promontory fort * [[Redoubt * [[Stockade * [[Star fort Fortification and siege warfare * [[Medieval warfare * [[Military engineering * [[Military history * [[Siege * [[Siege engine Notable experts * [[Henri Alexis Brialmont * [[César Cui * [[Bernard de Gomme * [[Francesco Laparelli * [[Mozi * [[Diades of Pella * [[James of Saint George * [[Fritz Todt * [[Menno van Coehoorn * [[Sébastien Le Prestre de Vauban * [[Maximilian von Welsch
Bibliography*Osadolor, Osarhieme Benson, "The Military System of Benin Kingdom 1440–1897]," (UD), Hamburg University: 200