 The now-ruined keep of the medieval Norman castle is commonly referred to as Clifford's Tower.Built originally on the orders of William I to dominate the former Viking city of Jórvík, the castle suffered a tumultuous early history before developing into a major fortification with extensive water defences.After a major explosion in 1684 rendered the remaining military defences uninhabitable, York Castle continued to be used as a jail and prison until 1929.