The Skull Castle was a Castle seen in the background of the episode, "Loyalty to the King." It was seen while Finn and Jake walked passed while carrying the head of an enemy that they had defeated off camera.
It appeared to be a giant skull with a bridge with no railing leading to it. It is white and bricks can be seen built into it. The door appears to have bars over it and there are two giant holes in the top, made to look like eyes.
- It is unknown if the skull used to be a being or if it was just built to look like a skull.