For thousands of years, the building of walls has played an essential role in shaping the world as we know it; from being used to monitor populations to controlling trade, they have often acted as borders of entire nations. In this episode, Howard Williams takes us through some of the most famous walls in medieval history and explores how two of the best-known linear earthworks in western Britain, Offa's Dyke and Wat's Dyke, have served to separate England and Wales. Howard Williams is a Professor of Archaeology at the University of Chester.