Yen Sid's tower is a location in Fantasia. Only the interior appears in the film itself, while other media has shown the exterior, most notably the Kingdom Hearts games, where it is called the Mysterious Tower.
In the original film where the tower makes it's debut, only the interior of the tower is shown. In the tower lives Yen Sid and young student Mickey Mouse. When using Yen Sid's sorcerer hat, the now Sorcerer Mickey creates a flood and the tower is filled with water. The tower is soon cleared by Yen Sid and his magic.
In the Disney Magix cereal commercial, Yen Sid's home was portrayed as a castle with an exterior, and showed its for the first time. In front of the castle was a bridge crossing over to a forest.
The accompanying CD story had Mickey inviting Donald, Goofy and Minnie to the castle to show off his abilities. During their visit, Donald accidentally unleashes a potion upon himself that turns him blue. The friends consult Ludwig Von Drake for assistance, though he claims it may take a while to create the remedy. The story ends with Mickey and Donald shaking hands, causing the former to turn blue, as well.
The tower's location was only truly seen in the universe of Kingdom Hearts. The tower is located on a floating isle while the isle itself is surrounded by hilltops and empty space. On the isle resides wildlife and the tower itself.
The tower was accessed by Mickey Mouse via mirror located inside his own bedroom. Inside the tower Yen Sid was creating a world for forgotten and rejected Disney characters. Mickey sneaks in and accidentally creates a monster and rushes out. He was later taken back into the tower and inside the ravished world. Near the end with the world saved he exits the world and forced out the tower by Yen Sid. The Tower is referred in the Graphic Novel as "Yen Sid's castle".