Canterbury Castle was probably founded soon after the Norman Conquest and certainly before the Domesday Book.. All that remains is the lower half of a large, oblong keep. The stepped splays behind the narrow window openings suggest an early date. The plinth and pilaster buttresses are typical Norman features. The entrance was at first-floor level in the northwest wall and excavations have uncovered a fore building.

The West gate is the only survivor of seven gatehouses in the wall. The fortress-like outer façade of the gatehouse, with machicolations overhanging the entrance and sturdy drum towers pierced by gun ports, contrasts with a more domestic townward front. Note the porticullis groove in the vaulted gate passage. The West Gate has survived because it housed the county gaol after the castle keep had become too derelict.