Lemmy's Swingback Castle, also known as Acorn Plains-, is the castle level of Acorn Plains and the first castle level of New Super Mario Bros. U and New Super Mario Bros. U Deluxe. The level is filled of lava pits and swinging platforms that make for some difficult jumping. This is the final level with the Flying Squirrel Pack.
The level is mostly supposed over lava and the player must navigate by jumping from platform to platform. The castle has one subsection that contains a Star Coin. At the end of the castle, there is a door that will take you to the castle's end. The boss, Lemmy Koopa, uses bombs and tosses them at the player while he rolls across the arena on his ball. After getting stomped on, he will throw bigger bombs with larger explosions.
- Star Coin 1: The first Star Coin can be seen while you jump across the first few sets of swinging platforms. It's easy to spot early on in the level, be sure to time your jump properly.
- Star Coin 2: The second Star Coin is sitting just underneath a platform hanging above lava. Using a Super Acorn, the player can follow a coin path that will guide you into collecting it and safely onto a platform.
- Star Coin 3: The last Star Coin can be found in a pipe in the middle of the lava. You'll see it as you cross several platforms near the end of the level. Once you're inside of the pipe, use a Super Acorn to glide towards the Star Coin and glide back upwards.