The stronghold of Mondain where he first crafted his Gem of Immortality is a place only accessible to the Stranger by use of a time machine, and serves as the background of the final confrontation of Ultima I.
Little is known about the location where Mondain finished the transmutation of his father's ruby into the Gem of Immortality, but it is in this place that the sorcerer made the crucial turning point from mortal to immortal, securing forever his boundless ambition. Over a thousand years after this event, Sosaria still suffered the horrors of the wizard's wrath.
It was only through the intervention of the time-traveling Stranger that this momentous scene was finally breached and the wrongs of the past undone. Wherever Mondain was at the inception of his gem's completion, it was here that the hero destroyed the wizard's work, and him along with it.
- Until the Stranger [G]ets the gem, Mondain is immortal and cannot be defeated.
- Within the fortress, the spells Create and Destroy can be used to erect or disintegrate sections of the walls, respectively (very useful to trap Mondain for the coup de grace); the Kill spell, on the other hand, has the effect of making Mondain stronger!
- Some versions of the final confrontation in Ultima I do not have any castle walls, presenting only a black screen.
- The stronghold of Mondain is visible in the introductory sequence of Ultima Online, as the beginning of the game replays the events of Ultima I in order to reveal how the multiverse in which Ultima Online takes place was created.