He was a prolific excavator of Romano-British sites, specialising in small-scale excavations, often just a single trench placed at a crucial point in a Roman fort which thereby established both the date and purpose of the fort. He excavated at Segontium, Chester, South Shields, Lancaster, Bath, Silchester, and Chedworth Roman villa. However his two major projects were at the Hod Hill Iron Age hillfort where he elucidated the Roman fort that was inserted into one corner, and then at Inchtuthil near Perth in Scotland, the legionary fortress occupied during Agricola's advance into Scotland. He also wrote a highly successful book on Roman Britain for the Penguin series.