Mushroom Kingdom U is a stage appearing in Super Smash Bros. for Wii U and Super Smash Bros. Ultimate, and it takes inspiration from New Super Mario Bros. U. Acorn Plains, Rock-Candy Mines, Meringue Clouds, and Slide Lift Tower are all featured in the stage.
Combatants fight on the base and/or platforms of the the stage. It is not a moving stage such as Mushroomy Kingdom in Super Smash Bros. Brawl. Kamek the Magikoopa will come around and cast a spell on the stage, changing its theme. It transitions similarly to the stage Castle Siege. It has four different forms to change from in a random order. The first form takes place on a high platform topped with grass. It has multiple soft platforms and has grassy hills all around it. The second form has two towers with a soft platform between them, and a soft platform above the right tower, all surrounded by a sea of clouds. The third form is a rock balanced atop a mountain. The rock tips right and left slanting the ground somewhat like Lylat Cruise. The fourth form consists of two slide blocks revolving around each other in a castle dungeon.
There are various things that appear occasionally regardless of the current form. Breakable ice stalactites fall from the sky. Water drops fall shortly before, indicating when and where they will land. A giant beanstalk may sprout up in the background with its leaves acting as platforms. Water Spouts will shoot upwards becoming usable as platforms. Some of the Water Spouts have Big Urchins causing damage to players on contact.A group of Bowser themed flying platforms will sometimes fly across the top of the stage. Stretch Blocks appear around the top half of the stage. They are soft platforms that can change their length. Nabbit also appears as a stage hazard, capturing players. It's possible break free if you mash buttons or if Nabbit is attacked. You can also K.O. him, if you attack him enough.
This stage is used in the event Enough with the Kidnapping. In this stage Peach has to go against Bowser and Bowser Jr two on one with a Boss Galaga infinitely respawning and only affecting Peach herself. Nabbit works exactly the same as regular gameplay (albiet appearing more) and is the only kidnapping element that can affect Boswer and Jr (Peach herself included) making him more of a neutral party than a hazard.
This stage originates from New Super Mario Bros. U, taking its main designs and themes and using them as platforms and obstacles. Acorn Plains is the first world of the game. It is a grassy area with a lot of hills and trees. The beginning form of the Mushroom Kingdom U stage takes place in Acorn Plains. Another form is located in Meringue Clouds. Meringue Clouds in New Super Mario Bros. U is the seventh world. It is high up in the sky and has very many clouds that can be used as platforms. Meringue Clouds, like all wolds in New Super Mario Bros. U, has a tower midway through the world. One of the Mushroom Kingdom U stage forms is in this tower. The tower contains a lot of Slide Blocks which serve as one of the form's main platforms in the stage. The tower's boss is Kamek who also makes an appearance in the stage. The last form takes place in the sixth world, Rock-Candy Mines. Rock-Candy Mines is a mountainous area with a few Spine Coasters and tracks. It also has multiple Wobble Rocks which is what the main platform of the Rock-Candy Mines form of Mushroom Kingdom U is.
The ice stalactites from Frosted Glacier, the beanstalk, water spouts, and Big Urchins from Sparkling Waters, and Stretch Blocks from Meringue Clouds are hazards or platforms in both Super Mario Bros. U and Super Smash Bros. for Wii U that also originated from Super Mario Bros. U.