Roper Klacks' home, laboratory, fortress and lair, located to the north of Riverwood. April wonders if it's the Arcadian equivalent of an RV. The enormous castle floats several feet off the ground, anchored by Lorhan, who is also the only one who can pull the steps to the castle down for April to enter.
The interior of the castle is composed of a large labyrinth to deter intruders, leading to an antechamber below the main chamber where April confronts Klacks. Upstairs is Klacks' laboratory, where he keeps his spell book, alchemical essences and cauldron.
The castle itself is probably enchanted to make it so light: once its anchor is freed, the castle is actually blown away on the wind April summons, all the way back to Marcuria. Lorhan dropped to safety in the farmlands before the castle eventually crashed somewhere outside the city.