Castle Oblivion

Castle Oblivion is a world found in the Realm Between in Kingdom Hearts: Chain of Memories, and is one of only two real worlds that appears in Chain of Memories. It also appears within cutscenes in Kingdom Hearts 358/2 Days, Kingdom Hearts Birth by Sleep, and a digital copy of the world appears in Kingdom Hearts coded. It was governed by Marluxia of Organization XIII, and consists of thirteen floors and twelve basements.

Kingdom Hearts: Chain of Memories
Our friends may be here...It was that thought that led us to Castle Oblivion, where cards rule all.
 * Sora's Tale I

A mysterious man gave Sora a card, and using it took us to Traverse Town—of all places!

It turned out the town was just a figment of our own remembrances, the first world of many that lie in Sora's heart.

And so our journey through these worlds begins.

Kingdom Hearts Birth by Sleep
Castle Oblivion was initially the world Land of Departure. After the defeat of Master Eraqus at Terra's hand, Master Xehanort summons the darkness to destroy the land. Aqua later brings a comatose Ventus to the Land of Departure to keep him safe. As Aqua walks into the dilapidated castle, she remembers a "secret" that Master Eraqus told her when she became a Master. He told her that he was the protector of the world and that if anything ever happened to him, she should seal the world's keyhole with the Keyblade to prevent darkness from taking over the world. She was also advised to recreate the world so that only she knew the structure and layout of it and that any unwelcome visitors would be "lost in oblivion".

As a result, she uses the Master Keeper to seal the world's keyhole and recreates the world into Castle Oblivion. She then leaves Ven in the newly-formed Chamber of Waking and promises to bring Terra back with her before waking him up. When Aqua departs from the building to go look for Terra, she turns around to look at her new home, completely transformed.

Between Kingdom Hearts Birth by Sleep and Kingdom Hearts: Chain of Memories
The castle is eventually found by Xemnas of Organization XIII, who decides to convert it into a second headquarters for the Organization. It is intended to be a place to research memories, an important topic to the Nobodies of the Organization, who have only their memories to form personalities with. Xemnas sends Xigbar and Xaldin to recruit more members for the Organization, as they need a large group in order to run the castle.

According to a flashback in Kingdom Hearts II Final Mix that takes place in between Kingdom Hearts Birth by Sleep and Kingdom Hearts, Xigbar and Zexion discuss the new castle and the Chamber of Repose, which house Aqua's armor and Keyblade, that Xemnas constructed in Radiant Garden while he was Xehanort, and which he frequently went to. Xigbar believes that Xemnas was looking for the complementary Chamber of Waking, and that this place might be somewhere in Castle Oblivion.

Eventually, a strange Nobody named Naminé appears in the castle and is found by the Organization. She soon becomes the center of their research, due to her strange abilities of memory manipulation. In order to further research, pods are developed that can be used to piece together memories. Xemnas entrusts the castle and Naminé to the neophyte Marluxia, but he soon plots to use this power in a bid to seize control over the Organization. While Marluxia and his associates, Larxene and Axel, control the top half of the castle, the senior members Vexen, Lexaeus, and Zexion control the basements. Throughout their appearances, they are shown to have a strong rivalry.

Kingdom Hearts: Chain of Memories
After defeating Ansem, Seeker of Darkness, 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. Although Marluxia and Larxene 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 can use Sora as a puppet against Xemnas. The scheme fails thanks to the meddling of Axel (who was actually on orders from Xemnas and Saïx to rid the Organization of traitors), who frees Naminé, and Marluxia and Larxene are destroyed at Sora's hand. Sora then uses one of Xemnas's 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 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; instead he sets out on the "road to dawn" in order to help while Sora is sleeping.

Kingdom Hearts 358/2 Days
After members of Organization XIII are sent to Castle Oblivion (which starts the events in Kingdom Hearts: Chain of Memories), Roxas learns from Demyx and Xigbar that the members assigned to Castle Oblivion have been annihilated. Xion later goes to Castle Oblivion seeking answers about her existence. Axel tries to stop her, but his efforts are futile. Later, when Roxas learns from Axel that Xion was born in Castle Oblivion, he becomes determined to investigate the place for himself. However, barely has he walked in the door when he is overcome by a strange weakness and passes out entirely, due to how close he physically was to Sora and Ventus.

During conversations between Axel and Saïx, Saïx asks Axel if he has found the Chamber of Waking, which Axel denies. Axel is sent various times by Xemnas to find the room, showing Xemnas's extreme determination to find the Chamber of Waking, yet he never succeeds, due to Aqua's previous doing.

Kingdom Hearts II
Roxas briefly sees the outside of Castle Oblivion in his final dreams, coinciding with Sora regaining the corresponding memories.

Kingdom Hearts coded
After defeating Sora-Heartless and returning to the data worlds, Data-Sora enters a data version of Castle Oblivion. He has meetings with the mysterious cloaked man, as well as various figures from Sora's memories. Like the real Sora did, Data-Sora uses World Cards to traverse the castle. One notable feature of this incarnation of Castle Oblivion is that the White Rooms do not change form completely; the design remains like the rest of the castle, although the shape of the room differs. Pictures of areas from the worlds are displayed on the walls. Near the end of the story, he meets Data-Naminé, who explains the reason of his existence.

Trivia

 * In the manga, the monitors through which the Organization watches Sora are "Nomura-brand", a reference to Tetsuya Nomura, the director of the Kingdom Hearts series.
 * The Castle Oblivion logo depicts stairways that resemble those of M.C. Escher's Relativity.

Notes and references
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