A spur was constructed on the northern side of the castle in the C13th after the 1216 siege and reflects successive attempts by military engineers to reduce the vulnerability of the northern defences from attack from the higher ground to the north. The bridge towards the spur and its associated tower (St John’s Tower) formed part of the elaborate post-siege strengthening of the northern defences, providing a key link in the communications between the castle and spur. This circular tower was under construction in the middle of the moat in the 1220s and the medieval tower was refurbished in the 1750s. (location accurate to the nearest 1m based on available information)