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Whirling Fortress is a move that can only be used by Bowser and Giga Bowser in Super Smash Bros. Melee, Super Smash Bros. Brawl, and Super Smash Bros. for Nintendo 3DS / Wii U. 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.
Flying Fortress increases the vertical distance of the move to about twice the distance of Whirling Fortress, but it offers less horizontal distance. Also, while the in-game description states that it doesn't deal any damage, it can actually deal minor damage to opponents. On a side note, the Dash Slash custom special move works well alongside Flying Fortress; Dash Slash can offer horizontal movement to help counter Flying Fortress's lack of horizontal mobility.
Sliding Fortress greatly increases the horizontal distance of the move, enabling it to travel as far as the entire Battlefield stage. However, the attack takes longer to start up, and it only delivers a single hit on opponents with reduced knockback, making it less effective to use as an out of shield attack.
Ludwig von Koopa does a move similar to Whirling Fortress in Super Mario World. In Mario Power Tennis, Bowser uses a Special Shot similar to Whirling Fortress, known as Spinning Shell Dash. Bowser uses a similar move in Mario Sports Mix, and in Super Paper Mario in one of his Stylish Move animations.
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