Dark Tower

When a Liche took full possession of Horance of Skara Brae, he turned evil, and forced the people of Skara Brae to erect a huge Dark Tower. The tower stood where Horance's house had been, connecting the small isle with the main island. Blackrock was woven into the walls to shield it from any harmful magic. The center of this dark building is the Well of Souls, where the Liche kept the souls of the dead trapped, feeding on them. 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 it remains as a sign of what once had been.

Trivia

 * Despite it being built partly of Blackrock, Rudyom's Wand won't do anything.