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You'll understand after playing Birth by Sleep then KH1, so spoilers can be avoided.
Castle Oblivion originated in Kingdom Hearts Birth by Sleep after Ventus feel into a comatose state. Aqua created Casltle Oblivion to have a place to keep Ventus safe. Castle Oblivion is also used as a headquarters for Organization XIII. In Chain of Memories, Sora, Donald, Goofy, and Jiminy Cricket search the Castle hoping to find Riku and the King, but end up losing their memories and having to fight the Organization instead. They are then contained in sleeping capsules in the castle until their memories return.
Sora, Donald, and Goofy sleep in 3 giant, white pods for a year in Castle Oblivion while Roxas works for Organization XIII.
In Chain of Memories, the trio only go through Castle Oblivion and that big grass field in the beginning of the game when they are looking for King Mickey and Riku.
Sora, Donald, Goofy, and Jiminy are seen walking down a long road, after the events of the first Kingdom Hearts. They come across a hooded man in a black coat who points them to a castle and informs them that "To find is to lose, and to lose is to find." Sora and the gang travel to Castle Oblivion and ascend from floor to floor, visiting memory-based Worlds in the form of cards.
No, Sora Donald and Goofy are in a different Castle
Sora, Donald goofy and Beast in times
Mickey Mouse , Minnie Mouse , Goofy , Scrooge McDuck , Jiminy Cricket , Cinderella , Winnie the Pooh , Donald Duck , Tinkerbell , et.al.
Sora and his friends Donald, Goofy, and Jiminy Cricket are walking down a winding path when a mysterious man dressed in a black, hooded coat appears. He directs Sora towards a massive fortress called Castle Oblivion and then disappears. When the travelers enter, they find the mysterious figure again. When Sora and his companions attempt to engage the figure, they find that their abilities seem to have no effect. The figure explains that they forgot all of their abilities when they entered the castle. He continues by creating a deck of cards made from Sora's and his friends' memories, and tells them that everything they encounter in this castle will be based on their memories. The higher they go, the more memories they will lose in the process. Meanwhile, floors below, Riku has been transported from the realm of darkness and fights his personal demons as he battles upward from the basement levels of Castle Oblivion.
Donald & goofy
Donald and Goofy are Sora's sidekicks thoughout the game. Donald specializes in Magic and Goofy in defence. They are very loyal allys
Donald and Goofy