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There are 15 secret castle stars which do not appear in regular courses.

[edit] Toad

Toad will give you three free stars just by talking to him. The following is a list of where to find all three of these stars:

1. In the basement in the room with Hazy Maze Cave
2. On the second floor just under the stairs leading up to the third floor
3. On the third floor next to the Tick Tock Clock.

[edit] Mips the Rabbit

After you have 15 stars and less than 50 stars a rabbit can be seen hopping around the basement. Catch him and he will give you a star. He will return after you gain more than 50 stars and can be caught again for another star. It is possible to catch him again, though he will not give you any more than two stars total.

[edit] Super Mario 64 DS only

After you get 5 stars, Boos will start appearing in the courtyard. They have eight red coins in them, so kill them to get all of the coins and the star.

After getting a certain amount of stars, glowing rabbits (white ones) will start randomly appearing in place of normal rabbits. After catching eight of them, you will get a key to the last door in the character switch room. Inside that is a star.

Another star is attainable only by Luigi. If you grab the flower in the mirror room, go through the mirror, and enter the door on the other side, you will find yourself in a blank room with a star.