The eighth world of New Super Mario Bros. ULTIMATE is Bowser's domain. It is lava themed. After defeating Ludwig in World 7, he gives up the key to World 8 and Mario sees Bowser Jr. with Peach. After a brief chase through World 8-1, Mario follows Bowser Jr. into the Battleship level where a duel takes place. This continues throughout the world until Mario gets to the castle. Bowser tells his son to return to the castle and orders his other kids to guard the castle. Mario defeats them. Bowser Jr. refuses to let Mario through to his dad's castle and tries to stop him in his airship, but can't do it. He retreats to his dad's castle and the battle starts. (More explained below) The world has two main paths, joining at boss battles, split after the battleships with odd and even levels.
Bowser's Castle 1
The only battleship level in the game requires the player to fight Bowser Jr. Mario starts by the dock, where three Mechakoopas try to attack him. Mario must defeat them and collect the Magic Wand, which makes its first appearance. It allows Mario to blast anything out of his way and manipulate items. Mario must pull the switch sticking out of the lava, making the bridge fall to the first ship. Bob-ombs and Fire Bros. try to stop Mario as he jumps from ship to ship, making his path difficult. Before the checkpoint, the player must fight a large Mechakoopa as a miniboss. His defeat opens the path to the checkpoint. Mario must continue in a section similar to the previous, but Bowser Jr. orders his troops to sink the ships. The ships have more enemies and sink rather quickly into the lava. After getting to the end, Bowser Jr. challenges Mario on the last ship. Unlike his siblings, which can be defeated by stomping, Bowser Jr. is immune to this attack. He attacks by throwing shells at Mario. Mario must grab them and throw them back three times, or just control the shells with the Magic Wand to double his attack power.
Bowser's Castle 2
Bowser's Castle 3
The tanks level has Boom Boom guarding the exit.
Bowser's Castle 4
Bowser's Castle 5
The fortress is owned by Kamek.
Bowser's Castle 6
Bowser's Castle 7
Castle Air Fleet
The air fleet's boss is Boom Boom.
Bowser's Castle 8
Bowser's Castle 9
Bowser's Castle 10
Bowser's Castle 11
Castle Tower 2
The second tower has Bowser Jr. as a guardian.
Rainbow Switch Palace
This is accessible by clearing the tower's secret exit, unlocking 8-10 and 8-11, which both open a secret route into the second tower. Its secret exit leads here.
Bowser's Castle 12
Bowser's Castle 13
Castle Tanks 2
The second Tanks level has twenty-four large Reznors controlling tanks, which fortunately is the majority of the level.
Castle Tower 3
The third tower is the only tower guarded by more than one boss, which in this case are Kamek and Boom Boom together.
Castle Air Fleet 2
Bowser Jr. commands the air fleet.
The castle is guarded by all of Bowser's kids, who must be fought simultaneously. They all use the same attacks from their castle fights.
Bowser Jr. controls the airship.
Bowser is the boss of the final castle, and he is much harder to defeat than in any previous game.
The player starts outside in the courtyard, which suddenly is lowered underwater, and Mario must get through the door before it floods. On top of the meteors, Fishbones, and Fire Bros. all over, Bowser Jr. pursues Mario halfway through. If Mario gets inside, the water recedes and he can touch the checkpoint. The next section has many Podoboos all over the course, and Mario must get past them as a huge cannon blasts deadly dark matter at him. After the second checkpoint, Mario will see three green pipes. The left one takes him through a maze of pipes, only to lead him back to that room. The middle one takes him to the roof, where the secret exit is. It also leads directly to Bowser, although this method is very hard and the player can choose to bypass this. The right pipe takes Mario to a long passageway with a mini battle against Bowser Jr., who can be defeated with a stomp. His defeat leads to a third checkpoint, past which are little trials. Getting to Bowser, Kamek comes inside and strengthens Bowser making him able to turn into a large robot. Mario must use cannon pipes to reach ledges to groundpound a switch on Bowser's head. After five hits, Bowser stumbles and falls on Kamek, who accidentally tries to use magic to save himself. He misses and accidentally turns Bowser into a seven-hundred-foot robot monster. He accidentally breaks Peach's cage, knocking her down. She tells Mario that they can get into Bowser's inside structure by blasting open his foot, which Mario does. Bowser Jr. follows Mario inside, who tries to reclaim Peach. If she is captured, Mario must start over. When they get to the top, Mario must hit a large switch, which activates self-destruct. He then has two minutes to get back out the door made in his foot. Bowser explodes along with his son, who are cast into space. Mario and Peach then head out of the castle and the credits roll.
The back door is accesible by getting the secret exit on World 8-12, and it takes Mario directly to Bowser.