Castle Oblivion

Castle Oblivion is a world found in the Realm of In-Between in Kingdom Hearts: Chain of Memories as the only real world in the game. 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.

Chain of Memories
Castle Oblivion was found by Xemnas of Organization XIII at this point, who decided to convert it into a second headquarters for the Organization. Marluxia was entrusted with control over it, but was secretly plotting to overthrow the Organization leader. After defeating Ansem, Seeker of Darkness, and restoring the damaged worlds, Sora, Donald Duck, and Goofy follow a path that leads into Castle Oblivion, which then forces them to fight through the castle to regain their memories. However, Sora's memories were being manipulated by a girl named Namine so that Marluxia could use Sora as a puppet against Xemnas. Meanwhile, Riku awakens within the basements of Castle Oblivion, and faces Organization members Vexen, Lexaeus, and Zexion to counter Marluxia's use of Sora, but they are all eliminated by Riku. Ultimately, the plan to control Sora 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. Riku is able to make it to the castle's top and discovers his friend's state, where he decides to depart from the castle to help while Sora is sleeping.

358/2 Days
Because the game takes place during Chain of Memories, Castle Oblivion is mentioned when Roxas learns from Demyx and Xigbar that the members assigned to Castle Oblivion were annihilated. Xion later goes to Castle Oblivion seeking answers about her existence.

Kingdom Hearts II
The Castle's inside was seen in the opening credits of Kingdom Hearts II, featuring Sora's battle with Marluxia, Riku's battle with Ansem, Naminé's placing Sora in the Memory Pod, as well as several other important events in Kingdom Hearts: Chain of Memories illustrated in full computer graphics. The outside of Castle Oblivion was briefly shown in Roxas's final dreams, as Sora's memories end as he opens the door.

Birth by Sleep
The castle's origin is revealed in this game; it was initially the Land of Departure, but after the demise of Master Eraqus, it was available for the darkness to take over. However, Aqua returns to the remains of the castle and recreates the world into Castle Oblivion to prevent darkness from taking over the world. The new castle and layout would also be designed to keep unwanted visitors from gaining full access to it. From there, Aqua places Ventus in the castle's Chamber of Waking and departs.