Canterbury was first walled by the Romans in the late third century, and the medieval walls — substantially rebuilt by Archbishop Sudbury in the 1370s — follow that same line. About half the original circuit survives, and a footpath runs along the top of the eastern and southern sections, giving a free aerial tour of the old town.
The walk is at its best from Dane John Mound round to the cathedral precinct: trees, public gardens, the bastions still in place, and good views over the rooftops to the cathedral towers. Allow 40 minutes for the full surviving stretch. The northern walls, which used to enclose the cathedral and the King's School, were taken down in the seventeenth and eighteenth centuries.