A detailed study of the masonry defences of one England’s most important Roman sites. Erected in c. 270 AD, the masonry walls of the Roman town of Silchester (Calleva Atrebatum; Hampshire, S. England) are part of the third system in a series of defensive works. They stand today to a height of almost 5m and are composed of up to seven lifts or stages, each consisting of a flint core and facing (now almost completely robbed away), capped by a string-course of large blocks and slabs that stretches across the full width (c. 3m) of the walls, formed of a wide variety of rock-types foreign to the district.