Castle Bowser is an enormous being able to utilise wonder power in Super Mario Bros. Wonder. Formed after Bowser gets his hands on the first Wonder Flower, Castle Bowser serves as a stand-in for Bowser's main castle.
Castle Bowser is a massive flying fortress formed from Prince Florian's own castle being assimilated by Bowser and his Koopa Clown Car. It has Bowser's face with glowing yellow eyes and pink pupils and lime green sharp teeth on the front of its green, dome-shaped bottom side and his horns (albeit green) on the sides. Four small pipes surrounding a massive pipe-like structure are seen on the fortress's bottom side, emitting purple smoke that corrupts the Flower Kingdom's environment. The top side resembles Bowser's usual castles with lime green highlights, and a spiralling structure similar to the top of many Russian castles can be seen with lime-green and black spirals. A second Bowser face, around a quarter the size of the fortress's "main face," that being Bowser’s true form, is between this spiralling structure and the castle. Two asymmetrical lime green horns are visible to the structure's sides.
In Super Mario Bros. Wonder
After Bowser gets his hands on a Wonder Flower, Bowser and his Koopa Clown Car assimilates Prince Florian's castle, grows to a massive size, and creates this massive flying fortress. He then uses his new powers as a weapon to conquer the Flower Kingdom, corrupting the area and populating it with nasty creatures.
The fortress itself serves as the final world of the game, with five levels listed below. No shops appear inside the fortress.
Missile Meg Mayhem, where Mario has to make his way through the entrance hall as Missile Megs get in his way.
High-Voltage Gaunt, which involves Mario trying to navigate through multiple electric currents.
Evade the Seeker Bullet Bills!, where Seeker Bullet Bills ambush Mario from multiple angles in their haste to attack him.
KnuckleFest Bowser's Blasting Beats, where massive hot Bowser hands attack at a certain rhythm.
The Final Battle! Bowser's Rage Stage, where Mario has to face off against the final boss after a Wonder Effect gauntlet.
Castle Bowser is eventually destroyed after Mario destroys Rage Stage Bowser at the end of the game and uses the Wonder Seeds, though the Koopa King survives the fortress's demise, flying off in his Koopa Clown Car after the staff roll.
Trivia
The fortress's design is similar to the Pigmaparoid boss from Star Fox: Assault, being a semi-sentient structure that is meant to wreak havoc.
Gallery
The fortress immediately after its creation, before it starts to wreak havoc.
