Extra Bosses around 70 if you are new.
Hollow Bastion- I think its when you turn into a heartless, if so go to the library, the part where you fought Riku the first time. You will see Kairi, Donald,and Goofy. After the cut scene you go to the art where you first arrived at Hollow Bastion and you will be atomatically transported to Traverse Town.
I think that is right, but if not tell me and I will find out.
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