continuous Roman (ancient Rome) defensive barrier that guarded the northwestern frontier of the province of Britain (United Kingdom) from barbarian invaders. The wall extended from coast to coast across the width of northern Britain; it ran for 73 miles (118 km) from Wallsend (Segedunum) (Wallsend) on the River Tyne (Tyne, River) in the east to Bowness on the Solway Firth in the west. The original plan was to construct a stone wall 10 Roman feet wide (a Roman foot is slightly larger than a standard foot) and at least 12 feet high for the eastern sector and a turf rampart 20 Roman feet wide at the base for the western sector; both were fronted by a ditch, except where the crags rendered this superfluous. At every ¹/₃ Roman mile there was a tower, and at every mile a fortlet (milefortlet, or milecastle) containing a gate through the wall, presumably surmounted by a tower, and one or two barrack-blocks. Before this scheme was completed, forts were built on the wall line at roughly 7-mile intervals and an earthwork, known as the vallum, dug behind the wall and the forts. Probably at this stage the stone wall was narrowed from 10 Roman feet wide to about 8 feet. The fortlets, towers, and forts continued for at least 26 miles (42 km) beyond Bowness southward down the Cumbrian coast.

Emperor Hadrian (Hadrian) (ruled AD 117–138) went to Britain in 122 and, in the words of his biographer, “was the first to build a wall, 80 miles long, to separate the Romans from the barbarians.” The initial construction of the wall took approximately six years, and expansions were later made. Upon Hadrian's death, his successor Antoninus Pius (138–161) decided to extend the Roman dominion northward by building a new wall in Scotland. The resulting Antonine Wall stretched for 37 miles (59 km) along the narrow isthmus between the estuaries of the Rivers Forth and Clyde. Within two decades, however, the Antonine Wall was abandoned in favour of Hadrian's Wall, which continued in use nearly until the end of Roman rule in Britain (410).

Hadrian's Wall was built mainly by soldiers of the three legions of Britain, but it was manned by the second-line auxiliary troops. Its purpose was to control movement across the frontier and to counter low-intensity threats. There was no intention of fighting from the wall top; the units based on the wall were trained and equipped to encounter the enemy in the open.