Bowser's Gravity Gauntlet

Bowser's Gravity Gauntlet is a galaxy in the game Super Mario Galaxy 2. It is located in World 4. The level is based entirely on gravitational forces. Most of the level involves sidescrolling, except for the beginning and the end. The level is covered in lava, but there is one part where Mario will need to swim through water to get through the level. Magikoopas are common enemies here and can summon Li'l Cinders at will. Fire Bars are placed throughout the whole level, even underwater. Fireballs also defy gravity because they can change direction in midair. When Mario reaches the end, Bowser will challenge Mario to a battle. Bowser battles in the same manner as in World 2, but this time he creates shockwaves and leaves a trail of fire.

This is the only Bowser level throughout both Super Mario Galaxy games where there is water to swim in.

Planets
NOTE: Unless otherwise noted, all names are unofficial.

Bowser's Castle
The level focuses on a large castle where there appears to be almost no normal gravity at all. The castle is surrounded by lava with fireballs jumping out of it. When Mario enters the castle, he will find more fireballs jumping out of the lava. As he progresses, he will switch to walking on the walls and eventually on the ceilings. Beyond these, there is an underwater section with Fire Bars and Cheep Cheeps. After this is a small outdoors section with Whomps, which leads to the other side of the castle with spinning platforms Mario must navigate to reach the door that takes him to Bowser.

Large Door Planet
A circular planet where Mario must Ground Pound three meteorites at three large locks in order to open the door leading to the next battle with Bowser. Additional meteorites will rise up from the ground whenever Mario Ground Pounds the previous ones to launch them at the locks on the large door. A Checkpoint Flag is also found here, along with two Crystals; one contains a Life Mushroom, the other contains some Star Bits.

Throne Planets
A small planet shaped like a star, in front of which is another planet that acts as Bowser's throne. When Mario first appears here, Bowser will be absent, but will quickly come crashing down to the planet. After talking to Mario, Bowser will send him to the Bowser Battle Planet.

Bowser Battle Planet
It is very similar to the first battle planet, except for the fact that it is more deteriorated. It is a small planet made out of small bricks and a few rocks with coins.

Green Star 1
The first Green Star in this galaxy is on a sunken pillar in lava near the entrance to the castle. Mario can backward somersault onto the pillar. Also, Mario can jump into the lava to bounce up onto the pillar and grab the Star.

Green Star 2
The second Green Star in this galaxy is at the end of the castle above a ? Block. Mario can backward somersault and Spin to get the star. It is ideal to get the star with Luigi instead of Mario.

Prima Official Game Guide description

 * "You've finally made it to the edge of World 4. Bowser lurks here, perfecting his schemes at the heart of his flying castle. Survive his gauntlet of tricks and traps, outsmarting ever-changing gravity fields. If you make it through Bowser's defenses, the Koopa King has no choice but to battle you for possession of a Grand Star."

Names in other languages
Bowsers Schwerkraftparcours