Arthuriana! ♥ And Celtic stuff in general. I'm also really intrigued by the possibility that the Welsh Prince Madoc and his followers may have ended up in the Americas at some point.
Though actually, the thing about the Middle Ages--and any period in history, for that matter--is how the people really lived then. Clothing, speech, relationships of different kinds, daily life for different classes, what they ate... Maybe it's because I'm a writer, but it just fascinates me.