Castle for Rent

For some reason, organized, blue-skinned warriors are wandering through the castle. When Incarnadine, the castle's owner, goes off to consult a long-distant brother, he finds himself trapped in the very low magic environment of Earth, only to realize the castle's problem is much, much worse than a few odd warriors.

Once upon a time, the king of a mysterious castle found himself out of place. The odd thing was that the time was the future and the place was New York City! Castle Perilous had been a universe unto itself until it started living up to its name. Now it is a gateway to thousands of universes, each increasingly bizarre and more bewildering. Behind every door, there is a room for every fantasy, and around each corner, a perilous show more journey for every inhabitant. With its population sent to otherworldly realms and demented demons taking their place, will anyone find their way home in time? The stone walls may be the only stable element in this fantastic fable, as time and place disappear before one's eyes. Pandora's box has never been so big! show less

A transformed demon, interdimensional portals, magical powers, and a sibling rivalry for the ages.... I read this as a kid and remembered it fondly. Now I wish I'd kept on remembering it - it didn't stand up well to re-reading.