He was a nobody by the name of Zexion. Apparently this castle was his previous home back when he was one of us and had a heart. He had been trying to rebuild this castle and this town. Before him and his idiot friend disappeared, he left me two things, one of which was this castle.

[So Kaiba, because he can respect another man's wish's, is restoring the castle to Zexion's plans. He just wants the castle, doesn't care what it looks like.]