How to get to Hollow Bastion?

In Kingdom hearts 1, I did all the other levels. Found all the keyholes. Done. Went back to all the levels to see if I forgot something. NOTHING. This is driving me up the damn wall. How do I get to Hollow Bastion? I went to see Merlin, he just asked if I wanted to practice magic. Went to talk to Cid, He just wants to sell Gummy stuff. What do I do?!?! AHHHHH.