The Nightmare Castle is a dreaded mountainous palace that is the lair of the Nightmare King and a location in Little Nemo: Adventures in Slumberland.
Nightmare Castle is a huge mountain surrounded by large sharp pointed rocks. It is the middle of the sea. It has a force field at the entrance, the only to get in is to go under it. There a prison chamber where the Nightmare King keeps his prisoners in giant reliquaries.
- The Nightmare Castle originally made an appearance as a giant icy pillar-like castle