York Castle is a fortified complex in the city of York, England. It consists of a sequence of castles, prisons, law courts and other buildings, which were built over the last nine centuries on the south side of the River Foss. 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.
The first motte and bailey castle on the site was built in 1068 following the Norman conquest of York. After the destruction of the castle by rebels and a Viking army in 1069, York Castle was rebuilt and reinforced with extensive water defences, including a moat and an artificial lake. York Castle formed an important royal fortification in the north of England.