The Fortress is a location found in Epic Mickey: Power of Illusion. It is where Oswald the Lucky Rabbit resides and where the saved toons can be found after they have been rescued. The fortress acts as a main hub of the game which allows access to all wings of the Castle of Illusion.
The Fortress Main Hall is directly in the center of the Castle of Illusion, and Oswald will keep you updated once you've saved enough toons to progress to another wing of the castle. Once you rescue Pinocchio, he also stays there.