In the manga Kingdom Hearts: Chain of Memories, much of the story takes place in Castle Oblivion.
Castle Oblivion is a mysterious structure controlled by the Organization. As Sora climbs through its floors, the castle’s rooms are shaped by memory cards, creating worlds based on places he has previously visited.
The castle’s central concept is the instability of memory. Sora’s memories are altered as he progresses, while Naminé’s unusual power allows her to manipulate the links between those memories. This makes Castle Oblivion both a physical setting and a symbolic representation of forgetting and reconstruction.
Castle Oblivion also appears in the wider Kingdom Hearts franchise, including the original video game Chain of Memories. It should not be confused with Hollow Bastion, another major location associated with Maleficent and later events in the series.