Castle Oblivion

Castle Oblivion (忘却の城) 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.

Organization Headquarters
The history of Castle Oblivion before Chain of Memories is still unclear, but 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 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.

In Kingdom Hearts II: Final Mix, Xigbar and Zexion discussed the new Castle and the Room of Sleep which Xemnas constructed while he was Xehanort, and which he frequently went to. Xigbar believed that Xemnas was looking for the complementary Room of Awakening, and that this place might be somewhere in Castle Oblivion.

Kingdom Hearts: Chain of Memories
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.

Kingdom Hearts 358/2 Days
Roxas, Axel, and Xion are shown to visit Castle Oblivion, though it is not yet known why.

Trivia
The need for the card system in Castle Oblivion is unknown. Though Marluxia claims that they are simply part of the castle, he claims that the trademark memory loss was due to the castle's influence as well, when in actuality it was Naminé's doing. It is possible that the card system was created by the Organization simply to disorient and manipulate intruders like Riku and Sora.