The castle of Newcastle was built during the reign, and at the command of William the Conqueror, under the direction of Robert Curthose, his eldest son, on his return from an unsuccessful expedition against Malcolm, king of Scotland,... As this building protected the bridge, and secured at all times a free pass for the English armies into Northumberland, it was a place of considerable importance.

From: T Fordyce, J. Sykes. Local records; or, Historical register of remarkable events which have occurred in Northumberland and Durham, Newcastle-upon-Tyne, and Berwick-upon-Tweed..., published 1867

This was a motte-and-bailey castle (wooden fort on an earth mound) at Monkchester, the 'New Castle', later replaced by a stone castle in the 12th century.