Kingdom Hearts Birth by SleepEdit
Castle Oblivion used to be The Land of Departure but when Master Xehanort left the world in ruins, Aqua transformed the castle and its outside into Castle Oblivion. She was instructed to do so by Master Eraqus if anything should ever happen to him. He told her to seal the world's keyhole to prevent darkness from taking over and to recreate the world so that only she knows the layout. This would leave unwanted intruders "lost in oblivion".
Kingdom Hearts 358/2 DaysEdit
Before 358/2 DaysEdit
The leader of Organization XIII, Xemnas, discovers the castle and claims it as his second headquarters for the organization. It was intended to be a place to research memory.
According to a flashback in Kingdom Hearts II, Xigbar and Zexion are having a conversation and Xigbar talks about Xemnas. Xigbar believes Xemnas is searching for the Chamber of Waking, believed to be somewhere in Castle Oblivion.Eventually, a strange Nobody named Naminé appeared in the castle and was found by the Organization. She soon became the center of their research, due to her strange abilities of memory manipulation. In order to further research, pods were developed that could be used to piece together memories. Xemnas had entrusted the castle and Naminé 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.
During 358/2 DaysEdit
During the events of Kingdom Hearts 358/2 Days, some members of Organization XIII are sent to the castle (which starts the events of Chain of Memories). The members assigned were Marluxia, Larxene, Axel, Vexen, Zexion, and Lexeaus.
After some learning of the members being anniliated from Demyx, Roxas is lead to believe his friend, Axel, was anniliated, too. However, he is relieved to see Axel is okay. 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, to which Axel replied that it was difficult to find since Castle Oblivion is huge and it is currently unknown if that room was ever found. It is also revealed that Axel would be sent various times by Xemnas to find the room, showing Xemnas's extreme determination to find the Chamber of Waking, yet he never could. Aqua, as per Master Eraqus's suggestion, had specifically designed the castle for its secrets to be known to nobody but herself. Thus, Aqua's design led Axel to be "lost in oblivion".
Kingdom Hearts: Chain of MemoriesEdit
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 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 coded Edit
After having defeated Sora's Heartless and with all the data in the journal reseted, Data Sora finds himself in the data version of Castle Oblivion. Here he is constantly taunted by a mysterious man in a black coat while exploring the castle. The castle uses world cards, like in Chain of Memories, in order to access incarnations of all the worlds he had visited. The worlds have multiple endings depending on how the player's actions and all endings can be obtained if the player chooses to go back. Instead of having the rooms that resemble the world, it is just a normal White Room with a different layouts for each world and has pictures of the real world on the walls. The illusions of characters Data Sora met believe they are in their respective world and are clueless that they are even illusions. Near the end of the story, he meets Data Naminé, who explains the reason of his existence.