Bamburgh Castle is reputed to be Lancelot's fortress Joyous Guard by Malory, though he does offer the castle at Alnwick (about 15 miles south of Bamburgh Castle) as an alternative.
Bamburgh Castle certainly existed as early as 547, a time that would allow an King Arthur connection. Before being called Bamburgh Castle, it had been called Din Guayrdi, one that quite easily suggests "Guard." However Malory's placing of Joyous Guard at Bamburgh Castle is not substantiated.
The castle in the King Arthur legend was originally known as Dolorous Guard and was owned by a wicked despot. There the despot remained until Lancelot single-handedly took the castle. He changed its name to Joyous Guard and made it his main residence.
Bamburgh Castle today is huge and is on a rock separated from the sea by dunes and grasses.