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I kinda beat the castle, but I went back to the castle instead of advancing. So how do I get back? Please don't say I have to redo the castle.

Go back to the part of Peach's castle with the slingstar that takes you to the castle. Next to it, there should be a pipe that takes you to the room where you beat the fake-bowser.

If you have any more questions, just look at the complete walkthrough here.
Locked.