Beginning in Super Mario Bros. 3, Mario must navigated various fortresses throughout each level. Fortresses are designed similarly to castles, but are smaller. In Super Mario Bros. 3, Bowser-like minibosses guarded the treasure of the fortress. Strangely, the treasure they guarded was a circle with a ? on it. In the first fortress of Mario 3, a Warp Whistle is accessible by flying above the area leading to the boss battle room. In Super Mario World, fortresses are guarded by Koopalings, and the only castle is Bowser's Castle.