Shadowguard

Shadowguard is the fortress of Minax the Enchantress, which resides in the Time of Legends, seen in Ultima II.

The castle has several interesting rooms, including a museum of Minax's triumphs. It is also inhabited by a host of terrible monsters, and protected by force fields which instantly kill intruders not wearing the Force Field Ring. Here, the Stranger found Minax, and killed her in combat, returning 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.