“White Thorn Castle”
Chastiau la Blanche Espine, Tour de la Blanche Espine
Its lord, Maten (Matain), was Arthur’s enemy. Lancelot, Bors, Gareth, and Bagdemagus, passing by this fine castle, found a hundred ruffians ill-using Mordred. The four knights rescued their companion.
On learning from him that Maten had attacked him simply for belonging to the Round Table, they fired the town, killed Maten, and burned the castle. At this time, Mordred seems to have been an earnest and promising young knight. At least, his villainy had not yet surfaced. Still, it is of course possible that he maligned Maten.
Blanche Espine appears to be on the way from Camelot to the land of the giant Mauduit. Where Mauduit’s land was I am not sure, but in non-Arthurian traditions, Cornwall is called a land of giants, and certain Arthurian stories bear this out, although not precluding the possibility that giants inhabited other areas as well.
Warbelow Barrow might be identified with La Blanche Espine. “Between Camelford and Launceston [Cornwall], on Wilsey Downs, is Warbelow Barrow, an ancient fortification of considerable size, in the centre of which is a large mound, popularly called King Arthur’s Grave.”