Castle Oblivion is the main location of Kingdom Hearts Chain of Memories and Kingdom Hearts RE:CoM. Despite it's secret location, the castle's lord, Marluxia, lures our hero, Sora, into his clutches.
There were 6 members of Organization XIII sent to Castle Oblivion. In order of number: Vexen (IV) Lexaeus (V) Zexion (VI) Axel (VIII) Marluxia (XI) and Larxene (XII). In order of alphabetical order: Axel (VIII) Larxene (XII) Lexaeus (V) Marluxia (XI) Vexen (IV) and Zexion (VI)
The castle was founded by Aqua, one of the infamous Keyblade Trio (Ventus, Terra, Aqua) of Kingdom Hearts: Birth By Sleep. It's original name; owner was Land of Departure, own my Master Eraqus.
Sora is not the only one who went through Castle Oblivion, Riku did also (Reverse Rebirth), and he didn't give a crap about Marluxia, he went for Vexen Lexaeus and Zexion. He was victorious.
This was a temporary hotel for Sora, and a permanent one for Ventus (so far.)
Marluxia: Welcome to Castle Oblivion, where to find is to lose, and to lose is to find.
Sora: I don't give a crap about some stupid castle!