The Castle that Never Was was once the leading base of operations for all Nobodies, where Xemnas and his legendary Organization XIII were stationed from the creation of the Castle to the demise of the Superior.
During Xemnas' final battle, the Altar of Naught was cut off, and it crashed below, destroying much of the massive castle. When Raxen first arrived on the World that Never Was, his plan was to restore the Castle to its former glory. For the duration of his first organization, he and the Lesser Nobodies he commanded rebuilt the Altar of Naught and much of the destroyed castle based on the plans Xemnas left behind after his downfall.
Upon the start of Organization Eternal, the Superiors of that organization moved their operations to Memory's Skyscraper below. Out of respect for Xemnas and the fallen Nobodies who died there, few but the Lesser Nobodies live there now. Most of the Castle is in disuse now, the Lessers using it more as shelter than as a headquarters. A few rogue human-forms may be found there, but all-in-all, the human-forms only come to pay respects at the enlargened Proof of Existence, or to hold meetings in Where Nothing Gathers.