From the Super Mario Wiki
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The Ghost Castle is actually the back-right tower of Bowser's Castle on Yoshi's Island. It is infested with many Boos, including rare ones such as Blindfold Boos, Boo Blocks, and Little Boos. Many chambers are dark and eerie, and some are even cavernous. Ghost Riders can help the Yoshis reach higher areas of the level.
At long last, the Yoshis reached the final page and the creepy castle. The castle was haunted by ghosts, but the Yoshis won the spirited fight. And Bowser was no match for the Yoshis, either. What a big baby!
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