Okay. I have beaten Riku and gotten the emblem peices. Now what?
Any links to an *GOOD* walkthrough will do good also. =)

Did you entered the portal? You have to fight a Behemoth, and then you close the keyhole

The keyhole closing doesn't come until you go back to Hollow Bastion (yes, you have to go back...). What you do now is put the embelm pieces into the door with the heartless symbol on it and then enter the lift stop. Then curse the day you ever picked up this game! Navigation is now going to become a nightmare, especially because if an enemy knocks you off a platform you'll fall all the way to the bottom and have to start over again.

I'd say head over to gamefaqs.com and check out ElectroSpecter's walkthrough. It saved me many times.

But he already beated Riku, but we don't know exactly wich battle, so.... we can't really help him