World 4-Fortress 1
From the Super Mario Wiki
World 4-Fortress 1 is the first fortress level and the fourth level overall of Giant Land in Super Mario Bros. 3. This level becomes accessible once the player clears World 4-3 and its own completion opens World 4-4.
The player starts at the bottom of some stairs, which lead to a narrow hallway with some Hot Feet. At the end of the hall is a sliding Thwomp that will move horizontally when the player gets near, followed by another sliding Thwomp shortly afterward. After the second sliding Thwomp are a group of Warp Pipes, the middle of which will take the player to an underground cave with some Dry Bones and the player can collect a Fire Flower (or Super Mushroom) in one of the ? Blocks. To leave the underground area, the player needs to find two Hidden Blocks and use them as platforms to get over the ledge to the right. If the player doesn't go down the middle pipe, they will have to avoid one falling and two sliding Thwomps before reaching the exit pipe from the underground area. Next, the player has to move through a long hallway with Hot Feet before reaching Boom Boom. Boom Boom can be defeated using similar methods as his previous fights, though Boom Boom will occasionally jump high in this fight. Defeating Boom Boom and collecting the Magic Ball will open up a gate nearby on the map.