Whirling Fortress

Whirling Fortress is 's up special move in Super Smash Flash 2.

Overview
When performed, Bowser retreats into his shell, where he rapidly spins around before getting back out. He can move very quickly horizontally while spinning, and if used in the air, he also gains height, much like 's Spin Attack. The move can hit opponents multiple times, with the first hit being a sweet spot dealing 14% damage and strong upward knockback to opponents who connect with it. The move can hit up to five more times with very weak knockback after this, with the first hit dealing 5% damage and the final four dealing 3% damage for a total of 31% damage if all hits connect.

When used in the air, this move is a decent recovery option, as it lets Bowser travel much faster horizontally and a small distance vertically, though it also leaves him helpless afterwards. On the ground, however, Bowser will not become helpless after moving off-stage with the move, though he will also descend slowly rather than rise. It also makes a decent out of shield option, due to its low startup lag and strong sweet spot. Additionally, if Bowser uses the move on the ground and moves off-stage with it, he can grab a ledge during the beginning of the move.

Origin
In the Mario games, starting with the original Super Mario Bros., Koopas retreat into their shells when jumped on, and can be used as an attacking spinning projectile when kicked, thrown, or jumped on again. In Super Mario Bros. 3, when the player jumps on any of the Koopalings during their boss battles, they will fly into the air while inside of their shell. Ludwig von Koopa also had a move similar to Whirling Fortress in Super Mario World.

Despite the above, Bowser himself never used this move, and in fact was never seen retreating into his shell in any Nintendo-developed game prior to the move's debut in Super Smash Bros. Melee. Like the Egg Roll and Green Missile, however, the move's concept later led back into the Nintendo-affiliated Mario series starting with an appearance as Bowser's defensive power shot in Mario Power Tennis, and Bowser using shell maneuvers is now a somewhat common occurrence.