This was a second castle of the Duke of Tintagil, ten miles from Tintagil.
It had "many issues and posterns out". This was where Gorlo´s was slain the same night Uther begat Arthur on Igraine in the sister castle of Tintagil. Arthur later battled and defeated the armies of King Ryons' brother Nero and King Lot in the field before Terrabil, where Lot met his death.
When Phyllis Ann Karr visited Tintagil in the late 1960's, neither the official guide nor anyone else had ever heard of Terrabil. John W. Donaldson places it near St. Kew, Cornwall, about midway between Camelford and Padstow. This seems more suitable than a few of Donaldson's other place identifications, especially since Mee, in The King's England: Cornwall, notes prehistoric earthworks and rumors of Arthurian castles. The best clue yet found, however, appears in the Survey of Cornwall. Carew identifies Terrabil as adjacent to the town of Launceston, and says that it was from the regard of the castle's triple walls that men so named it.
The site appears to be a little farther than ten miles from Tintagil - although not twenty miles, and miles might have been longer in Malory's day, or Arthur's.