Dark Tower

When the Liche took full posession of Horance of Skara Brae, he turned evil, and forced the people of Skara Brae to errect this huge Dark Tower, where once his house had been, connecting the small isle with the main island. Woven into the walls is Blackrock, which shields off any harmful magic. The center of this dark building is the Well of Souls, where the Liche kept trapped the souls of the dead, feeding on them. And every night at midnight, the Liche fed on the astral power of the ghosts of the inhabitants of Skara Brae, while undead creatures guarded his domain.

The tower lost its power, when the Avatar destroyed the Liche and the Well of Souls in Ultima VII, but the tower remains as a sign of what once had been.

Trivia

 * Despite it being build partly of Blackrock, Rudyom's Wand won't do anything. Very strange... On the other hand, it wouldn't be very smart to let the whole tower blow up.