Although the first defenses built by humans to protect themselves from their enemies were surely temporary, the earliest extant archaeological evidence concerns permanent works. Archaeologists date stone walls and towers at
Jericho, clearly built to repel human assailants, to about 8000 or 7000 b.c. By late Neolithic times, settlements of stone or mud brick in the Fertile Crescent were built with the outermost dwellings joined together to form a defensive perimeter, the rooftops serving as observation and firing platforms similar to those of Native American pueblos in the southwestern United States. By historical times, cities were routinely surrounded by walls of stone or brick sufficiently high to discourage escalade, topped by a walkway or fighting platform and supported by towers projecting above and forward of the wall to cover it with defensive fire. The walkway was shielded by an outer wall, crenellated, or notched, to provide cover for observation and missile fire, and often featured overhanging galleries from which projectiles and unpleasant substances could be dropped or poured on an enemy below. Ditches gave the wall greater height and made the approach to it more difficult. So close was the relationship between fortifications and civilization that the Chinese characters for wall and city are the same.
Permanent linear defenses also appeared in antiquity, the most famous being Hadrian's Wall and the Antonine Wall of Roman Britain and the Great Wall of China. Some of these, notably the Great Wall of China for much of its length, were normally used not to stop attackers outright, but to channel their movements. Similar, though less massive, defenses called letzi were used by the Swiss to control access to populated valleys. Armies on the march defended their camps with ditches and earthen ramparts, often topped with timber palisades and firing towers. Roman legionaries were renowned for their industrious work with spade, mattock, and axe; fortified Roman camps were famous for their size and sophistication, and some formed the nucleus of later cities.
Forts and cities protected by Roman stone walls remained after the fall of Rome, but many in the West were abandoned. The collapse of
Charlemagne's empire in the ninth century saw the emergence of a decentralized political and economic system based on local chieftains' strongholds, generally timber palisades atop earthen mounds. This type of fortification, closely associated with the rise of
feudalism, reached a peak of sophistication in the motte and bailey fortress of the eleventh century, with a high conical keep for the lord and his retinue and an outer enceinte, or bailey, for lesser retainers.
Motte and bailey castles in economically peripheral areas were progressively abandoned, and surviving examples were replaced by permanent stone castles with an inner keep and high curtain walls and round corner towers surrounded by a wet or dry moat. The entry way, an obvious point of vulnerability, was protected by gate, drawbridge, portcullis, and outer works, which became increasingly elaborate with time. Walls and towers grew progressively higher, and sophisticated provisions were made for the defensive firepower of crossbows and, later, gunpowder weapons. Machicolations, overhanging stone galleries, and hoardings, their temporary wooden equivalent, provided platforms to defend the base of the wall with dropped objects and missile fire.
The development in Europe of efficient siege artillery and the explosive, subterranean mine turned the medieval castle's strengths into weaknesses: unobserved "dead space" at the bases of round towers represented opportunity for gunners and miners, and high walls brought down by cannon fire filled the ditch with rubble for assault parties. In the late fifteenth century Italian engineers responded to the challenge with earthen ramparts, which absorbed cannon balls; with walls and towers cut down in height, protected from escalade by a ditch and from battery by a raised outer slope, or glacis, swept by defensive cannon and harquebus fire; and with the trace italienne plan, which featured projecting angular bastions of "arrowhead" shape with straight walls, swept by artillery firing in enfilade from protected casemates. (Interestingly, the Japanese developed the angle bastion independently.) Progressively reinforced with outlying ravelins, hornworks, and crownworks by Dutch and French engineers, the trace italienne fortress with stone-clad earthen ramparts and sunken profile set the standard for permanent fortress design from about 1500. In the eighteenth century it was challenged by "vertical" systems, which employed layers of heavily casemated guns piled several stories high to drive the enemy back with sheer firepower.
Both systems were rendered obsolete overnight by heavy rifled siege guns in the 1860s and gave way to works of reinforced concrete, sunk in the earth and defended by steel-armored gun ports and turrets. These were made obsolete in turn—or so it seemed—by highly explosive, armor-piercing shells in the early days of
World War I and gave way to trenches, bunkers, and other semipermanent earthworks protected by barbed wire entanglements. The lesson was reinforced by the presumed failure of the Maginot line in 1940, but in fact, pronouncements of the demise of reinforced concrete fortifications were premature. These proved a useful means of magnifying the power of the tactical defense in
World War II and are used today for strategic command-and-control centers and ballistic missile silos.
The advent of effective gunpowder weaponry in the sixteenth century produced a renaissance in field fortification that culminated in the elaborate lines of countervallation and circumvallation of formal eighteenth-century European sieges. Thereafter, temporary fortifications waned in importance until the rifled musket forced infantry to ground in the final stages of the
American Civil War. The tendency of European professional soldiers to deny the importance of field fortifications foundered in the face of fire from breech-loading rifles, machine guns, and quick-fire artillery in the
Russo-Japanese War and received a terminal setback from the same forces in World War I.
Since World War II, cannon- and machine gun-armed bunkers and pillboxes of earth or reinforced concrete with overhead protection from artillery fire, connected by communications trenches and protected by minefields and barbed wire (in static operations) and foxholes and quickly scraped out and sandbagged shelters for men and vehicles (in mobile warfare)—all carefully camouflaged, at least when built by professionals—have been a constant feature of land warfare and are likely to remain so for the indefinite future.
Christopher Duffy, Siege Warfare: The Fortress in the Early Modern World, 1494-1660 (1979); Anthony Kemp, The Maginot Line: Myth and Reality (1982); Charles W. C. Oman, "Fortification (1100-1300)," in A History of the Art of War in the Middle Ages, vol. 1 (1924).