What happens in Kingdom Hearts Re Chain of Memories

After the first KH game at the very end (after the credits) You see Sora, Donald, and goofy in a large field. Well the beginning of CoM you see them leave the field and end up in a place called 'castle oblivion' were you go through the rooms of the castle losing and regaining memories. In Reverse Rebirth of CoM is Riku's story when he entered castle oblivion. Instead of going up the castle he goes down the castle and ends up in the field at the end. His story is happening the exact time Sora is also in castle oblivion.