Palace of Blackthorn

Castle Blackthorn was a horrible place from whence Lord Blackthorn ruled over Britannia in the absence of Lord British. The castle itself was intentionally built as a maze to confuse intruders, and full of foul creatures, members of the Oppression, and traps.

It was built on the newly emerged volcanic island of Terfin after Blackthorn was manipulated by the Shadowlords. Interestingly, the position of the castle in the middle between the three volcanoes corresponds with the location where the Avatar retrieved the Skull of Mondain from the seabed in Ultima IV, indicating an evil spot.

The Avatar entered it in Ultima V, to retrieve the Crown of Lord British. After the end of the dictatorship, the castle was left to rot, but not for long; the mage Sutek made it his home, hoping to conduct his experiments in peace. The Avatar met him in Ultima VI while passing through the castle, to find the balloonist with his plans. Shortly later, Sutek accidentally turned himself to stone when fighting against some warriors. His statue was dumped into the sea, and the castle torched.

The Gargoyles used the stone to erect their own city, and so the castle ruins had vanished by the time of Ultima VII.

Trivia

 * Entering the castle without the Black Badge in Ultima V was a really bad idea...
 * Batlin still describes the ruins in The Book of Fellowship, which was written some years prior to Ultima VII, giving a rough point in time for when the ruins vanished.