Windemere

Description
Located on a mountainous island in the seas northeast of the Drylands, the fortress of Windemere was rewarded to the dark sorceress Elistaria and her mercenary cohort Thrud for their service to Lord Blackthorn and used as a stronghold for the Oppression. With no hewn paths through the treacherous mountains, access to the keep required the use of either a grapple to scale the peaks or powerful magic.

Daemons guarded the dark castle, watching over the high-ranking members of the Oppression stationed within. The Avatar visited this foreboding place in Ultima V at the behest of the archmage Flain for the purpose of retrieving a Black Badge to facilitate the infiltration of the Palace of Blackthorn.

The island only appears in Ultima V.

Trivia

 * A sign at the entrance to Windemere warns "Beware, Rodents of unusual size". Likely a reference to The Princess Bride.

Ultima V: Lazarus
In the Lazarus remake of the game, Windemere had been once the keep of a member of the Great Council called Windemere. Blackthorn had attacked the keep and killed all its inhabitants due to suspicions of Windemere's treasonous activities. After giving the keep to Elistaria, the Oppression started using it to train dark mages. Beside the daemons, skeletons also guarded the keep.