From the Super Mario Wiki
World 2-Fortress is the fortress level of Desert Land in Super Mario Bros. 3. It is accessed by clearing World 2-2 and its own completion unlocks World 2-3. This level marks the debut of the Thwomp and Boo enemies in the Mario franchise.
The player starts at the top of a small set of stairs with Dry Bones at the bottom. Continuing down the hall, the player will come across Thwomps and more Dry Bones, as well as a Boo after the second Thwomp. Past the Boo is a Warp Pipe that will take the player to the floor above, which has a Thwomp over a row of spikes. Afterward is a series of Thwomps the player must dodge in order to reach the door at the end of the hall. The next room has platforms covered with spikes that continuously raise and lower, along with two Boos placed above. The door after the fourth platform will also raise and lower accordingly, and will take the player to Boom Boom's room when they go through it. Defeating Boom Boom by jumping on his head three times will clear the fortress level and unlock a gate below on the map to a Warp Pipe and a Toad House.