It is unknown precisely when the Wall was abandoned, or even if it was abandoned wholesale, rather than piecemeal. The notion that the Romans all marched off in AD 410, leaving Britain and sailing ‘home’ is scarcely tenable any longer. By the 5th century, a significant majority of Roman soldiers were probably British in origin. Excavations at Birdoswald suggest continuity of settlement from the late Roman period through to post-Roman times, with a large timber hall constructed over the demolished fort granaries. Other sites hint at continuity, with farms and hamlets constructed on the remains of milecastles and forts used as medieval and post-medieval farmsteads.
Further reading: Symonds and Mason 2009