There are three keys in the Skull Castle.
1. The Bone Key - opens the door to the lich and the backdoor do the "treasure room".
2. The Labyrinth Key - opens the door at the top of the elevator in the "maze".
3. The Dungeon Key - opens the door to the lower floor of the castle.
The Bone Key and the Labyrinth key look the same so that you need to identify them if you want to know which is which. Or, you can use the Bungleau method
The Dungeon Key is different.