Shadowguard

Shadowguard is the fortress of Minax in the Time of Legends, visited in Ultima II.

The castle has various rooms, like a museum of Minax' triumphs, but at the same time, it is full with the most horrible monsters, and protected by force fields, which instantly kill an intruder not wearing the Force Field Ring. It was here where the Stranger found Minax, and killed her in combat to return the timeline to its normal state.

It is possible that the castle shown on the back side of the game box is supposed to be Shadowguard.