Mario's Rainbow Castle is Mario's board in Mario Party. It's a castle that takes place in the sky. It use to have rainbows until they mysteriously disappeared. By that time, the name Rainbow Castle was forgotten.
Toad and Bowser are at the end, on the top of a castle. Talking to either of them, or landing on a happening space, will cause the castle's top to spin, switching from Toad to Bowser, and vice versa. If you reach Toad, you have to pay 20 coins for a star unless you don't have more than 20 coins. If you reach Bowser, he'll force you to buy a Ztar for 40 coins; even if you don't have enough coins, all of your coins will be taken.
After becoming a Superstar, all the Stars everyone collected with collide into a huge Star. The huge Star will then create new rainbow for the castle and one huge rainbow bridge at the end of the board. 1st, 2nd, and 3rd place characters would climb the bridge. 4th place character would try to climb the bridge but slips falls off the board, then Lakitu appears and use his fishing pole the catch the falling 4th place character.