Built around 1086 by William the Conqueror as one of three royal castles guarding the road from Dover to London, Canterbury Castle's keep was once among the largest in England. Today it is a roofless square shell of stone, ringed by a small public lawn just inside the western city wall. The walls inside still show the marks of fireplaces, doorways, and the original spiral stair.
After it lost its defensive importance in the late Middle Ages the building did time as the county jail and a coal store, which is part of why it has been so badly worn. Free to visit; allow fifteen minutes. Combine with the Dane John Mound and the southern walls walk.