Sir Sadok passed this castle on his way to Arbray Castle, while escaping from Mark’s ambushment at Tintagil. I would guess Lyonesse Castle to be on the border between the land of Lyonesse and the rest of the Cornish peninsula, and Arbray to be a castle in the heart of Lyonesse.
At the northwest point of Land’s End, modern Cornwall, are
the ruynes of an auntiente castle .. vpon a loftie craggie rocke, where yet appeare the ruined walls and forlorne trenches.
The site is right for Lyonesse Castle.
Le Morte Darthur | Sir Thomas Malory, 1469-1470