World 7-Fortress 1
From the Super Mario Wiki
World 7-Fortress 1 is the first fortress level and the seventh level overall of Pipe Land in Super Mario Bros. 3. The level becomes accessible upon completing World 7-4 and its own completion unlocks World 7-6.
The player starts in a large room made up of Brick Blocks. Next to the stone platform, the player can find a Switch Block in the fourth block on top to the right, which will turn all the Brick Blocks into coins and reveal a secret doorway on the stone platform. If the player falls and enters the door below, then re-enters the door they came from, the player will return to the room with all the Brick Blocks reset.
If the player goes through the secret door, they will be brought into a room with a Warp Pipe on the ground, which leads to a bonus room with a Mega ? Block containing a Tanooki Suit. Entering the Warp Pipe in the wall (which leads to a room with lava and four ? Blocks - the one on the ledge above the door holding a power-up), then entering the door afterward, will take the player to a long hall which the doors in the first room also lead to. This room contains Roto-disc orbs, candle bases for Hot Feet, and Stretch blocks, but the obstacles and enemies themselves do not appear. Directly above the second Roto-disc orb from the left of the room is a Warp Pipe the player needs to fly up to. Entering the pipe will lead to Boom Boom, who is able to fly and dive at the player in this battle. Defeating Boom Boom will open up a gate near the World 7- 1 level on the map.