World 4-Fortress 2
From the Super Mario Wiki
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World 4-Fortress 2 is the eighth level and the second fortress of Giant Land in Super Mario Bros. 3. It becomes accessible after clearing either World 4-5 or 4-6 and its own completion unlocks World 4-.
The player starts at the bottom of some stone stairs, which ends at a pool of lava. To get across, the player needs to move across a group of broken up platforms made up of Donut Blocks, with a ? Block containing a power-up in the middle of the area. Afterward is a narrow hall with some Dry Bones and a Brick Block containing a P Switch. The Brick Block can be used as a platform to reach a hall leading to Boom Boom, though hitting the P Switch will reveal a door to a secret area.
If the player enters the secret door, they will be brought to the bottom of a vertically long area made up of several Warp Pipes and Arrow Lifts. The room also wraps around, meaning if the player moves through the left side of the screen, they will appear on the right and vice versa. Using the Arrow Lifts and Warp Pipes, the player can reach another pipe at the top that will drop the player into a room with some coins and Hidden Blocks that can lead to a Brick Block containing a 1-Up Mushroom. Going through the pipe on the floor will take the player to a room with a Giant ? Block containing three 1-Up Mushrooms, and if the player is powered-up with a Super Leaf, Tanooki Suit, or P-Wing, the player can fly to the left to reach an area with many coins. Going through the Warp Pipe to the right will take the player back into the fortress near Boom Boom.