How do you get to the keyhole in Kingdom Hearts 1 Hollow Bastion?

I was being dumb and I left the place where all the princesses were (I already defeated Riku) and Now I'm back to the main hall where that wrap around staircase is? I've went everywhere I can think of 3 times and I can't find the place where the princesses are! Please help me! And please try to be as detailed as possible. Those lift stop places are confusing :/