From the Super Mario Wiki
World 6 is the sixth world in Super Mario Bros. It is set once again at nighttime, and is the last world set at nighttime. It features four levels as usual, with two ground levels, one level at heights, and one castle level. No new enemies are introduced in this world.
||World 6-1 is the first level in world 6. It features the same enemies found in World 4-1: a Lakitu, Spinies, and Piranha Plants. The player is required to jump over several pits and maneuver through small mazes of bricks that are tightly arranged. Pipes with protruding plants must be lunged over to survive this level.
||World 6-2 is nothing much different from what the player has already encountered. Enemies include: Goombas, Koopa Troopas, Koopa Paratroopas, Buzzy Beetles, and Piranha Plants once more. This level also features a secret underwater area (which has Bloopers and Cheep-Cheeps as enemies) that can only be accessed via a special warp pipe. Once the player touches the flagpole, the level ends as always.
||This level is set at the heights once more, and is similarly built like previous levels. Bullet Bills and pits are the only enemies encountered in this level. Orange platforms are much smaller and require skillful maneuvering to land on in this level.
||This is the final level in World 6, thus it is a castle level. It is the exact same castle found in World 1-4, only this time more firebars and podoboos are found here. The player is able to maneuver through this level the same way as before since it is familiarly designed. A False Bowser (its true form this time is a Blooper) is once again encountered, and when defeated, a Mushroom Retainer is found. The player must go on since Princess Toadstool has yet to be found.
Mario about to defeat False Bowser.