Whirling Fortress (or 'the Fortress' as it's usually called) is Bowser's Up Special and third jump. Known for it's defensive and maneuvering attributes it adds to Bowser's game, it fills several 'holes' in Bowser's otherwise slow metagame. Several unique properties make it highly beneficial to play:

• It has very little start-up (5 frames), during which Bowser is invincible, and has relatively little cooldown lag.
• One cannot get shieldgrabbed if one cancels a Fortress directly into another.
• It hits throughout most of its animation (on the ground)
• It allows Bowser to move quickly and suddenly in one direction
• It has high knockback on the ground, and inflicts high damage in the air. The first frame of attack in the air also has high knockback. However, it's power was nerfed in Brawl, so it is not as useful at KO'ing as in Melee.
• It has high priority
• It can be used to edgeguard quickly and efficiently. Fortressing off the side of a stage leads directly to an edgehog

The above properties make the Fortress a versatile move. It can also be used as a damaging and effective replacement to Bowser's quick, yet laggy, roll. Can be jump-canceled out of a shield to discourage overly aggressive opponents. It can be used to clank just about anything with decent timing.

This special attack is shared by Giga Bowser, both in his boss appearance in Melee and in his playable appearance in Brawl. It gives him lots of invincibility frames and it's almost impossible to attack him while he is using this. In Melee, Giga Bowser's Fortress also gave him much more vertical recovery when used in midair.