From the Super Mario Wiki, the Mario encyclopedia
Whirling Fortress is a move that can only be used by Bowser and Giga Bowser in the Super Smash Bros. series. Bowser and Giga Bowser can activate this move whenever a player presses Up and simultaneously. Bowser (or Giga Bowser) conceals himself inside his shell and spins around with it in the air. If it hits any of the other players, they take a relatively large amount of damage, depending on how long they are in contact with the spinning shell. The attack can also be used on the ground, instead of just in the air. If used in the air, it will put Bowser and Giga Bowser into a helpless state after it finishes.
In Super Smash Bros. Melee and Super Smash Bros. Brawl, Whirling Fortress only delivers up to two hits when it's used on the ground, but it delivers up to 11 hits when used in the air. In Super Smash Bros. for Nintendo 3DS / Wii U, while Giga Bowser's Whirling Fortress remained unchanged, Bowser's Whirling Fortress can deliver up to 8 weak hits on the ground, with the final hit delivering the most knockback, making the attack more effective to use as an out of shield attack, and safer to use against shields.
Whirling Fortress received two custom variants in Super Smash Bros. for Nintendo 3DS / Wii U, which are Flying Fortress and Sliding Fortress. However, only Bowser can use them, as Giga Bowser can only use the default Whirling Fortress.
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