Mysterious Tower is a world which appears in Kingdom Hearts II, Kingdom Hearts Birth by Sleep, Kingdom Hearts coded, Kingdom Hearts 3D: Dream Drop Distance, and Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-. The tower serves as the home of the legendary ex-Keyblade Master, Yen Sid and a repository of great knowledge accumulated by the mighty wizard.
The Symphony of Sorcery originated from the Mysterious Tower, before turning into a sleeping world that appears in Kingdom Hearts 3D: Dream Drop Distance, the second world to be based on Disney's 1940 feature film, Fantasia. Also, like Timeless River and Country of the Musketeers, Symphony of Sorcery is set in the past, during a period where Mickey was in pursuit of knowledge to become a Keyblade Master.
Both serve as starting points for Sora and Mickey's quests in Kingdom Hearts II and Kingdom Hearts Birth by Sleep, respectively. Although it is considered part of Twilight Town in Kingdom Hearts II, like Dive to the Heart and Betwixt and Between, it is its own world and not part of the other.