World 4-Fortress 2

 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. This level introduces P Warp Doors.

Layout
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 Lifts, 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 Switch Block. The Brick Block can be used as a platform to reach a hall leading to Boom Boom, though hitting the Switch Block 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 Directional 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 Mega ? Block containing three 1-Up Mushrooms, and if the player is powered-up with a Super Leaf, Tanooki Suit, or Magic 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.

Glitch
If Mario enters a pipe on the left side of the screen from the side, the screen can scroll down and after a few seconds Mario will get to collect invisible coins and make some jump blocks appear. There's also another invisible pipe that can be entered and it will allow Mario to exit the level as if it were a non-fortress level. Finishing the level this way will not allow a bridge to be open; Mario won't get any access to the castle he has to enter to get to Iggy's Airship; the only way to get out of World 4 will be to use a Warp Whistle.

Enemies

 * Dry Bones
 * Piranha Plants
 * Boom Boom (boss)

Level statistics

 * Power-ups: 2 (Super Mushrooms or Fire Flowers)
 * 1-Up Mushrooms: 4
 * Secret areas: 1, split between three rooms