Dibella's Watch adds an entire new continent to Oblivion. It comes complete with a major city, castles, towns, villages, farms, monasteries and all the good stuff that makes up a Medieval landscape. There's also fighting to be had and quests to be made!
The island has many castles for the local lords of the small shires to live in. This is Stembrook Castle, which guards the borderlands of civilization in the west.