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|Origin||Kingdom Hearts: Chain of Memories|
|Stories||Kingdom Hearts Legacy|
|Theme Music||"Castle Oblivion"|
"The 13th Floor"
|Battle Music||"Forgotten Challenge"|
Castle Oblivion is an In-Between Realm world which appears in Kingdom Hearts: Chain of Memories, and is the only real world in the game. It is controlled by Marluxia of Organization XIII, and consists of thirteen floors and twelve basements.
In Birth By Sleep, you start in the Land of Departure. Towards the end, Aqua locks the Keyhole, and it turns into Castle Oblivion; however, in Chain Of Memories it is known that it was eventually found by Xemnas of Organization XIII, who decided to convert it into a new headquarters for the Organization. It was intended to be a place to conduct research on memories, an important topic to the Nobodies of the Organization, who have only their memories to form personalities with. Xemnas sent Xigbar and Xaldin to recruit more members for the Organization, as they needed a large group in order to run the castle. Eventually, a strange Nobody named Naminé appeared in the castle and was found by the Organization, soon becoming the center of their research due to her strange abilities. In order to further research, Xemnas also developed pods that could be used to piece together memories. The castle and Naminé were then entrusted to the neophyte Marluxia, but he would soon plot to use this power in a bid to seize control over the Organization. While Marluxia, and his associates Larxene and Axel controlled the top half of the castle, the senior members Vexen, Lexaeus, and Zexion controlled the basements. Throughout their appearances, they are shown to have a strong rivalry.
After defeating Ansem and restoring the damaged worlds, Sora, Donald Duck, and Goofy find themselves in a grassy field with the King's dog, Pluto. They follow Pluto along the path to Castle Oblivion, and as soon as they enter the castle, they forget all of their skills and are forced to rely upon the castle's memory-based card system. Though Marluxia and the other Organization members tell them their memory loss is simply the nature of the castle, it is revealed that the mysterious girl Naminé is actually the one reshaping Sora's memories, so that Marluxia and Larxene can use him as a puppet warrior against Xemnas. The scheme fails thanks to the meddling of Axel, and Marluxia and Larxene are lost to the darkness at Sora's hand. Sora then uses one of Xemnas' pods so that Naminé can restore his memories, and is eventually moved to Twilight Town.
Meanwhile, Sora's friend Riku awakens from the darkness within the lowest basement of Castle Oblivion, where he faces his memories of Maleficent and the remaining darkness of Ansem. Though he initially sets out to destroy the indelible Darkness in his heart, with the help of DiZ, Naminé, and King Mickey, he is able to eventually embrace both the Light and the Dark, achieving balance. The three basement members, Vexen, Lexaeus, and Zexion, seek to recruit Riku in order to counter Marluxia's use of Sora, but Riku denies their requests and vanquishes them. Riku eventually defeats the Ansem in his heart, and decides not to have Naminé seal it away completely by erasing his memories; instead he sets out on the "road to dawn" in order to help while Sora is sleeping.
The Castle may be the home of many of the Remnants of Organization XIII that Xashowd is searching for.
An entity known as Dinax currently wanders the halls of Castle Oblivion. Whether or not he will run into Kexon (or later on, Xashowd) is currently unknown.
Castle Oblivion is also the place that young Axane ran to when she heard of her sister's murder. She never really explored the castle, and never met anyone there, though she thought she heard voices every now and then. Axane was recruited to La Lutte Pour Presque Rien by Xiggie, who found her in the Organization's briefing room. It is unknown if she was working on something while staying there, and she remains silent about her stay in the castle.
During KHL I, Xashowd eventually finds a laboratory in the basement levels and recognizes it as the one where Vexen and Xemnas oversaw the Replica Program. After meeting up with Kexon, he begins to use the room regularly as a base of operations. In KHL II, Xashowd has managed to collect all of the Remnants of Xigbar and, with the help of Hikaru, revives Xigbar from the Remnant. Xashowd stays with Xigbar for a few days, catching him up on what happened after he was defeated by Sora and telling him of the fight against Lord Kami and his latest encounters with Raxard. They then head to Twilight Town to meet up with the remaining former members of La Lutte to discuss what happened after the fall of Lord Kami.