Hadrian's Wall

Hadrian's Wall was a Roman frontier work of the early 2nd cent. running 70 miles from the Tyne near Newcastle to the Solway west of Carlisle. Commenced at the behest of Hadrian on his visit to Britain in 122, the wall was originally to consist of a running barrier fronted by a ditch (except on the crags of the central sector), with a gateway defended by a fortlet every mile (milecastle) and two watchtowers (turrets) between each pair of milecastles. Apparently a frontier, it was designed to supervise not to deny movement. The line could not have been held against a concerted attack; in the event of a crossing Roman forces would concentrate to the south to expel invaders.

A Dictionary of British History. Ed. John Cannon. Oxford University Press, 2001.