The Guardian Fortress, based on the island of Terfin, is the Guardian's center of power in Britannia during Ultima IX. The outside of the fortress is cut out of the rock, looking like a huge Guardian-head, with the mouth acting as the entrance. High walls surround it as protection against intruders. Inside is a huge complex, where dozens of Wyrmguards live, protecting the central base of the Guardian. The biggest room is the throne room, where the Guardian is located when on Britannia, together with his Mirror of Darkness.
The Avatar first entered the fortress after being summoned (and subsequently cast out) by the Guardian. Later, the Avatar entered it through a secret passage to reach the throne room and prepare for the final fight with the Guardian.
- There are a number of inconsistencies regarding the fortress. For one, it is on the map, yet the people don't know of the Guardian. Also, it looks totally different from the ending of Ultima VIII (it had shrunken considerably).
- Concept art from the game identified the fortress as the "Tower of Exultation." While it is possible that this was originally meant to be the name of the place, it is identified as such during the course of the game.