Instructions
Enter the door on either side of the main staircase in the entrance of Princess Peach's castle. Both doors lead to the same place.
Open the door with the keyhole at the end of the hallway. You can open this door if you have defeated Bowser once and have at least eight Power Stars.
Follow the hallway. Take the right pathway at all three forks.
Open the door on the right at the end of the path. The room contains two pillars.
Press the "Z" button while jumping to ground pound both pillars. This causes the water in the moat to drain out.
Jump in the hole in the now-empty moat. This starts the "Vanish Cap Under the Moat" course.
Go down the large slide at the beginning of the level. The platform at the bottom of the slide is narrow. It is easy to go too fast and fall into the bottomless pit just past it. Hit the platforms sticking out of the slide to slow your descent.
Follow the platforms to the end of the course. This part of the course is straightforward and should pose no difficulty.
Jump on the blue switch at the end of the course. You can now get the blue boxes scattered around the game world.