Twinsy Tropics Dungeon
From the Super Mario Wiki, the Mario encyclopedia
The Mario Bros are first taken to the dungeon after fighting Big Pokey and Tower Power Pokey. After being knocked out by a cannonball shot from Bowser's Castle, the trio are captured by the Koopalings and Chargin' Chuck Corps and imprisoned in a cell in the dungeon.
After a quick escape courtesy of Paper Mario, the trio find themselves with no Command Blocks, with them all having been taken earlier by the Hammer Bros. standing guard. At this point, they must hunt down the Hammer Bros. and use a mixture of First Strikes and counter attacks to defeat them, gaining back one Command Block or Bros. Attack after each battle.
After getting their abilities back, the trio come across a Toad who explains how the Toads are being forced to mine Fire Ore for an unexplained purpose (which was later revealed to be a fuel source for Neo Bowser Castle). They also free the Paper Goomba they came across earlier in exchange for a deck of Battle Cards. Before they can escape, they battle Roy Koopa and Wendy O. Koopa, and notice a gate that can be opened with a Red medallion.
Later in their adventure, the trio revisits the dungeon after facing off with Papercraft Bowser Jr. on Mount Brrr and getting the Red Medallion they needed earlier. They come across the Paper Goomba from earlier, who betrays them and hands the medallion over to the new Fire Bro wardens.
After a quick hunt for the medallion via fighting the Fire Bros. littered around the dungeon, the party opens the mysterious gate seen earlier, which leads to a new part of the dungeon filled with conveyor belt and Horned Ant Troopers. They then follow this path out to a new area of Twinsy Tropics, which leads to Bowser's Castle.