York Castle in the city of York, England, is a fortified complex comprising, over the last nine centuries, a sequence of castles, prisons, law courts and other buildings on the south side of the River Foss. The now-ruinous 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 York, the castle suffered a tumultuous early history before developing into a major fortification with extensive water defences.