A castle in Scotland, embedded in a rock off the coast.
It was approachable only by a bridge guarded by a horrible hag wielding a scythe. The castle contained an enchanted white shield, guarded by a dragon, which prevented its bearer from death. Arthurís Sir Fergus learned of the castle from a dwarf, and he ventured there to obtain the shield. He emerged with it after slaying both the hag and the dragon.
There is an actual castle called Dunottar in Kincardineshire.