When using Whirling Fortress, Bowser retreats into his shell and spins rapidly while damaging nearby. It is similar in execution to Donkey Kong's Spinning Kong, being a move that causes the user to quickly spin around as a mostly-horizontal recovery.
While unique to Smash Bros., it is similar to how Koopas retreat into their shells when jumped on in the Super Mario series which, can be held and thrown or kicked by the Mario Bros. as a spinning attack. In Super Mario Bros. 3, when the player jumps on any of the Koopalings, they will fly in the air inside their shell. Ludwig von Koopa 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 also has used the move in New Super Mario Bros U.
In Melee, Whirling Fortress is known for the defensive and maneuvering attributes it adds to Bowser's game, which fills several holes in Bowser's otherwise slow metagame. As a result, it is widely regarded as the only reason Bowser is playable competitively in Melee. Several unique properties make it highly beneficial to play:
The above properties make Whirling Fortress a versatile move. It can also be used as a damaging and effective replacement to Bowser's quick but laggy roll, jump-canceled out of shield to discourage overly aggressive opponents, and clank with an opponent's move with decent timing. However, the ground attack can only deal up to two hits, and if any of the hits get blocked, Bowser will be highly vulnerable to punishment by his opponents, which includes getting grabbed. As a result, the ground attack is not safe to use against shields.
In Brawl, the move is relatively unchanged, with the exception of the aerial version granting slightly more height.
In Super Smash Bros. 4, pressing the special move button repeatedly during the move will give Bowser marginally more height (similarly to Dr. Tornado in said game), and is capable of grabbing ledges from behind. It also travels much faster and further than before. As an attack, the ground attack now delivers multiple weak hits (up to 8 hits, with the last hit delivering the most knockback) instead of two hard hits, making its use as an out of shield option even more reliable. The attack is also safer to use against shields, as the multiple hits make it harder for opponents to punish Bowser; they can still take the full impact of the final hit if they drop their shield too early.
This special attack is shared by Giga Bowser, both in his boss appearance in Melee and in his playable appearance in Brawl and Super Smash Bros. 4. It grants him multiple frames of invincibility, making it difficult to attack him out of the move. In Melee, Giga Bowser's Fortress also gave him much more vertical recovery when used in midair, and the ground attack can deliver multiple hits to nearby opponents; in Brawl, however, the ground attack only deals up to two hits, reducing its overall damage output.
Special Move customization was added in Super Smash Bros. 4. These are the variations:
Fortress hogging is a technique in Melee that allows Bowser to grab onto the edge after simply using his Whirling Fortress on the stage. Fortress hogging is performed by timing the Whirling Fortress so that the end of the animation allows Bowser to fall past the stage and in turn, grab on to the edge. However, on stages where there are sloped edges, like Yoshi's Story, Bowser will automatically Fortress hog onto the edge regardless of the position he is facing, given he is not facing away from the stage after falling.
Fortress hogging can be useful to quickly edgehog a recovering opponent, and allow Bowser to reach the edge faster than he normally would by walking or running.