Dure Roche, Rocca Dura
In Perlesvaus, the castle is occupied by Brian of the Isles, Kay, and Meliant when they invaded Britain and waged war against Arthur.
In the prose Tristan, it is a castle owned by Arthur, where the king and Guenevere held a tournament in the heyday of Arthur’s reign, between Arthur and the kings of Scotland and Ireland.
[Tristram] won the tournament of the Castle of Maidens that standeth by the Hard Rock.
From this it appears that the Castle of the Hard Rock either was near to or identical with a Castle of Maidens. Roche Dure or Hard Rock may have been an earlier or an alternative name for it.
Sources
Perlesvaus | Early 13th century
Prose Tristan | 1230-1240