And you can go and see it for the princely sum of £1 if you turn up within the last hour of opening. They won't let you up onto the ramparts though which will probably be the biggest disappointment of Mosh's entire life. Probably.
William the Conqueror, him of 1066 fame, built it although it's really the walls at the back of this picture that are Norman, the rest has been added and altered over the last 900 years or so.