The castle was first known of in 1096 when Yves de Vescy was Baron of Alnwick. Since 1309 it has been owned by the Percy family, the Dukes of Northumberland. It was built to defend England's northern border against the Scots, especially the border reivers.
The castle is situated in parklands designed by Lancelot Brown ("Capability Brown") and Thomas Call in the 18th century. Special exhibitions are housed in three of the castle's perimeter towers. The Postern Tower has an exhibition of the Dukes of Northumberland and their interest in archaeology, including frescoes from Pompeii, relics from Ancient Egypt and Romano-British objects. Constable's Tower houses displays about military forces raised to repel Napoleon's planned invasion, also the Northumberland Fusiliers Museum.
The Castle was used as a location in the film Harry Potter.