Totally Remake.
Castle Oblivion is castle that where Organitation XIII in there, and this castle have many ilussions from Sora's memories. Each level have a world from Sora's Memories like Traverse Town, Neverland, Wonder Land, and many more. So if you want to get to next floor, you must complete one world level.
Same as the GBA version, the gameplay is using Card system. There aren't any differents for the card system. But, i really don't like the gameplay. Sometime when fighting with a boss and you ran out of cards, that can be annoying.
About the graphics, I'm not seeing any problems. Still same as the first game, colorfull.
If you want to enjoy this game, you should play it for the stories. The reason I buy this games, just because the stories.
One more thing. If you haven't play this game or the GBA version, you may don't understand the Kingdom Hearts 2 stories.