Standing proudly on a basalt crag overlooking the North Sea, Bamburgh is among England's largest inhabited castles, with an estate that spans nine acres. Bamburgh Castle also ranks among Britain's oldest, as there has been a fort on the site since about the 5th century AD, when the site was probably the capital of the northern British kingdom of Bernicia. The Vikings and the Normans also saw the strategic importance of the rocky ledge, and altered the fortress to suit their needs. Many of the Norman alterations form the basis of the current structure; the oldest surviving complete building in the castle is the keep, which was built in 1164.