Castle Oblivion

Castle Oblivion, often abbreviated as "C.O." by members of Organization XIII, is a world introduced in Kingdom Hearts Chain of Memories. It is located within the Realm Between, though a digital copy of the world appears in Kingdom Hearts coded. It was governed by Marluxia, and consists of thirteen floors and twelve basements. After Marluxia's defeat, the Organization abandoned the castle as they lacked the manpower to keep it operational. Hidden within the castle is the Chamber of Waking, where Ventus sleeps; no one but Aqua can locate the Chamber, as she was the one who locked the castle to keep Ventus safe.

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.

Settings and Areas
The interior of Castle Oblivion consists of simplistic white marble architecture with infrequent plinths, wall-carving, and sculptures made with a flower motif. It is divided into many s which are shaped by the memories of those exploring them through the use of World cards and Map cards; though this second part may be due to the Organization's experiments with memory. Between each of the White Rooms is an Iriguchi Hōru (入口ホール) and Deguchi Hōru (出口ホール) for each of the thirteen floors and twelve basements. The Entrance Halls contain "s" which allow quick travel between the floors. The castle also contains a Poddo Rūmu (ポッドルーム), housing the pod within which Sora, Donald, and Goofy were put in hibernation; it is located on the thirteenth floor.

The thirteenth floor and the first basement each having a meeting room the Organization use; though only the top floor has a way to monitor the other floors, via a crystal ball. Naminé is kept in the top floor meeting room. The basement meeting room is dimly lit, and usually where Zexion can be found. Vexen additionally has a lab of some sort, where he was working on the replica project. Marluxia is also fought in sort of pocket dimension, where he freely shapes it to suit his needs; Sora seals the entrance, causing it to disappear.

In Kingdom Hearts Re:coded the interior of the castle is five separate white rooms modified to match the basics layouts of ', ', ', ', ' and '; images from each world appear in poster-esque fashion on the walls.

Data-Sora battles Data-Roxas in the Kanashimi no En (悲しみの園), a platform of stained glass disconnected from the rest of the world, reminiscent of the Stations of Dive to the Heart.

The interior is implied to be incredibly vast, to the point one would get lost in it, as was the intention of Aqua; the Organization compensate by using Dark Corridors. Axel has complained that searching the castle is next to impossible, stating it's "like counting grains of dust in a room of sneezing people".

Kingdom Hearts Birth by Sleep
Castle Oblivion was originally the Land of Departure. After the events at the Keyblade Graveyard, Aqua brought a comatose Ventus to the Land of Departure, which had been destroyed in a storm of darkness by Master Xehanort. In order to keep Ventus safe, Aqua follows Eraqus's prior instructions to seal the world's Keyhole with his Keyblade and recreate it as a labyrinth that only she could navigate, using a means devised by prior guardians. She leaves Ventus in the castle's newly-formed Chamber of Waking and promises to bring Terra back with her before waking him up. As Aqua leaves to search for Terra, she looks back on her transformed home.

Between Kingdom Hearts Birth by Sleep and Kingdom Hearts Chain of Memories
The castle is eventually discovered 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 with which to form personalities. Xemnas sends Xigbar and Xaldin to recruit more members for the Organization, as they need a large group in order to run the castle. Despite rumors among the Organization that Xemnas intends to locate the Chamber of Waking, to complement the Chamber of Repose built under Hollow Bastion, they never do.

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 the latter 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.

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, Roxas learns from Demyx and Xigbar on Day 26 that someone assigned to Castle Oblivion have been annihilated.

On Day 255, Axel went in Castle Oblivion to get rid of any evidence of the Organization's presence as the castle would no longer be used as a research facility/second headquarters. Xion goes to Castle Oblivion on the same day to seek answers about her existence. Axel tries to stop her, but his efforts are futile.

On Day 296, when Roxas learns from Axel that Xion was born in Castle Oblivion, he becomes determined to investigate the place for himself. The next day, Roxas was assigned to go there and hunt down the impostor. Roxas used the opportunity to investigate Castle Oblivion. Axel chose to tag along claiming that he knows the place better than anyone else in a black coat. However, barely has Roxas 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 actions.

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's 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. As 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 for his existence.

Kingdom Hearts 3D: Dream Drop Distance
Ventus continues to sleep within the Chamber of Waking, following the end of Sora and Riku's Mark of Mastery exam. Upon Sora's return to Traverse Town, he smiles from Sora's happiness.

Kingdom Hearts III
After being brought back to the Realm of Light by Sora, Aqua immediately sets off to find Castle Oblivion in order to free Ventus. Accompanied by Sora, Donald, and Goofy, Aqua uses the Master's Defender Keyblade to unlock Castle Oblivion's Keyhole, which reverts it back to the Land of Departure.

Kingdom Hearts II Final Mix
The Transport to Remembrance has the same aesthetic as the halls of Castle Oblivion.

Trivia

 * The need for the card system in Castle Oblivion is unknown. Although Marluxia claims that they are simply part of the castle, he also claims that the trademark memory loss was due to the castle's influence as well, when in actuality it was Naminé's doing.
 * In the manga, the monitors through which the Organization watches Sora are "Nomura-brand", a reference to Tetsuya Nomura, the director of the  series.
 * The Castle Oblivion logo depicts stairways that resemble those of M.C. Escher's .
 * The sequence when Aqua transforms Castle Oblivion back into the Land of Departure uses a technique called . This technique is used on the castles of many Disney Parks for their nighttime shows.

Notes and references
🇰🇲