From the Super Mario Wiki
Mushroom Heights, or Acorn Plains-4, is the fourth level of Acorn Plains in New Super Mario Bros. U. It is the first athletic course, and the first course to introduce Goombrats and Balloon Baby Yoshi (although the same Baby Yoshi can be acquired just before Mushroom Heights on the world map). This is the first course of the Acrobat Pack in Boost Rush. This course is unlocked by completing Yoshi Hill, and this course in turn unlocks Lemmy's Swingback Castle upon completion.
The course mainly comprises of moving Mushroom Platforms, such as ones that move up and down, or ones that tilt forward and backward. The enemies are mostly Piranha Plants, including the pipe and big variety, though the course does introduce a new type of Goomba, the Goombrat, Goombas with stems on the top of their head. They act like red Koopa Troopas, as they turn around on the edge of ledges.
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