Durham Castle is the ancient palace

Durham Castle is the ancient palace of the Prince Bishops of Durham and lies at the northern end of Palace Green opposite the cathedral. It is situated on the site of a fortress built to the orders of William the Conqueror on his return from Scotland in 1072. Waltheof, the Saxon Earl of Northumberland undertook the work of building the castle for William but over the years a succession of Prince Bishops have added important sections to the great building.
The present castle is dominated by the Keep which although the most imposing part of the castle is in fact the least historic. In the tradition of the Norman Motte and Bailey castles the keep is situated on a mound and was first erected in the fourteenth century during the episcopacy of Bishop Thomas Hatfield. Over the centuries the keep fell into a ruinous state but was rebuilt in the 1840s as a sleeping quarters for students when the castle became Durham's University College.
The older and greater part of the castle is situated around a courtyard to the west of the keep. The courtyard is entered from the gatehouse near to the site of the castle moat. The moat was crossed by means of a draw bridge just outside the Gatehouse. Primarily the work of Bishop Pudsey (1153-1195), the Gatehouse underwent some alterations during the episcopacies of Bishop Tunstal (1530-1559) and Bishop Shute Barrington (1791-1826).