Neil Fairbairn (in his Traveller’s Guide to the Kingdoms of Arthur) says the site is also known as Cadon Barrow. It is the grave of Cador, an Earl of Cornwall and close friend of Arthur. Guinevere was supposed to have been living at his house/castle when Arthur met her.
In the Dream of Rhonabwy (in the Mabinogion) his name is spelt Cadwr, and he is named as the man responsible for arming Arthur as he goes into battle.