The Nightmare Castle is a dreaded and towering, mountainous palace that is the lair of the Nightmare King and a location featured in the 1989 animated adventure fantasy movie Little Nemo: Adventures in Slumberland based on the 1905 comic strip Little Nemo in Slumberland.
The Nightmare Castle is a huge mountain surrounded by large sharp pointed rocks. It is the middle of the sea of oozing, living black water that represents all the darkness and fears of every dreamer. It has a force field at the entrance, the only to get in is to go under it. There is a prison chamber where the Nightmare King keeps his prisoners in giant pillar-shaped, crystal-like reliquaries.
The Nightmare Castle appeared as the final world in Little Nemo: The Dream Master, as the sixth and penultimate stage in Nemo and as the seventh and final stage in their Sailor Moon remake Sailor Moon Crystal: The Dream Master.
- The Nightmare Castle originally made an appearance as a giant, icy, pillar-like castle in the 1987 animated pilot film version of Little Nemo: Adventures in Slumberland (also known as "Nemo").