Dunottre
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. Arthur’s 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 a historic fortress called Dunnottar Castle on the northeast coast of Scotland. It is uncertain when it was constructed, but believed to be dating back to the Early Middle Ages. It’s built on a peninsula, surrounded by the North Sea on three sides. Saint Ninian established a place of worship on the site where the castle stands, around 400 AD.
Source
Fergus | Guillaume le Clerc, 1200-1233