Walkthrough:Kingdom Hearts Re:coded/Darkheart3/Castle Oblivion

Castle Oblivion
Okay, so you're in this strange castle, with a card, a guy in a black coat, and a mouse wait, no mouse. Heas up to the door, and select the Destiny Islands card. Oh! I forgot! You don't know about the card system yet! At the start of the world, some weird guy gave you a World Card. When you go up to the door, you'll be shown all the World Cards you possess. When you use one, you will be taken to an illusion-based version of that world. Depending on the choices you make there, you'll be presented with a different ending card. Collecting all the Ending Cards will result in a special item. So go up to the dorr and use the Destiny Islands card.

Normal Ending
Talk to Selphie, and agree to help her. Then talk to Wakka and do the same. Talk to Tidus, and defeat him, then talk to the cloaked man.

Alternate Ending
Talk to Selphie and say No. Do the same to Wakka. Then go and save Tidus.

Extra Ending
Talk to Selphie, then found the shiny object and examine it. Do the same with Wakka, and examine the second shiny object.