This world starts out with a Buster Beetle (new enemy) running toward Mario. A stomp will defeat the Buster Beetle it easily. On the brick formation, the wooden block contains a power-up, and a Chain Chomp to the left, which are somewhat prominent in the level. The second one guards the way to another power-up. There are no special secrets, except a Starman hidden in a long row of ? Blocks. Also, the Nipper Plants here can only be defeated by a swipe from Raccoon Mario's tail.
Alternately, if Mario has a P-Wing, he can enter a pipe, and then fly through narrow corridors to get a Music Box. There is also another pipe exit that Mario can use to gather four 1-Ups. Then, he can fly up and reveal a P-Switch which, when pressed, makes two giant numeral 3s made of coins (a 3 and 4 in the Game Boy Advance version) appear, which is a reference to the game's title (Super Mario Bros. 3, and Super Mario Bros. 3: Super Mario Advance 4 in the Game Boy Advance remake).
This underground level can be very confusing. After the pipe is entered, there are two routes. Falling down will give the lower route, while bouncing along the Note Blocks and climbing a block staircase will give the upper route. See below for summaries of each.
The upper route is the hardest to get to, but it pays off. Mario must leap on the Jump Blocks and hop on the blocks to get to the pipe, guarded by two Koopas he must dispose of. After entering the pipe, there are plenty of slopes and Goombas to slide on. When a pipe is spotted, Mario can duck into it to unlock a secret room. The jutting block has a power-up with which to break the blocks and get to the big ? Block that has three 1-Ups inside. Outside, none of the ? Blocks contain anything but Coins, but the pipe nearby leads to the Goal Panel.
If Mario took the lower route, he should duck into the middle pipe at the bottom to find himself underground, where Buster Beetles throw Ice Blocks, which can be picked up and thrown back. With the combination of the Beetles, Piranha Plants, lack of power-ups and Venus Fire Traps, this part might be considered difficult. All the pipes are solid except the last, which leads to the Goal Panel.
This level is the only in the game where Mario can find the Kuribo's Shoe. By ducking into the pipe, Mario is transported to the bottom part of the stage, where, if a Goomba in a green shoe is seen, Mario should hit a block under it to bop it out, letting him hop into it. While wearing the Kuribo's Shoe, Mario can defeat any enemy with a stomp. But the Shoe cannot be used if the Goomba is defeated with a jump or a fireball, as it will defeat the Shoe, too. In the first part, there is only one secret: above the Nippers, in a row of blocks, a 1-Up is concealed. However, only Shoe-clad Mario can grab it without being damaged. Hopping in the pipe against the wall will bring Mario to the third part of the level, where the Shoe is very, very useful. The Shoe will disappear when the Goal Panel is hit.
This fortress might be very tricky. The only secret is that if Tanooki or Raccoon Mario races back and forth along the first corridor (with a Thwomp in an alcove that must be avoided) then takes off, he can fly up past two blocks and a ? Block until he gets to a pipe that will take him to a secret room. Inside, he must fly up to reveal three 1-Ups.