Basement

Levels
The levels in the basement include: Hazy maze cave, Boo's Mansion, Wet dry world, Castle Secret Star Thwomp's Fortress, Castle Secret Star Frosty Fludd, Castle Secret Star Secret of the Sky, and the 2nd Bowser Level.

How to get there
By defeating Kamek and getting a special Shine Sprite, you can go into the basement.

Extra Details
You can find a door that leads you into the lake at the front of the castle. There is a platypus in a toad costume that checks the water quality of the castle. It is still unconfirmed whether it is really a platypus. The basement has the most hard to navigate levels, like Hazy Maze Cave, Boo's Mansion, Wet Dry World.