You begin the level by climbing up the first section, jumping on several platforms while avoiding lava underneath as well as enemies. Bowser can be seen as you go, blocking your way unless you shoot him, at that case he will climb up until he finally reaches a small room blocked with blocks so you cannot attack him further. Once you managed to make Bowser reach there, all you can do is to finish climbing up the castle and go to the next section.
The second section has Bowser running away from the player once the section begins. You can only chase him by progressing through this section, which has several bottomless pits (despite having "blood" that you can normally swim in) and enemies.
The third section has you climbing up ladders to reach the top part of Bowser's castle while avoiding enemies on the way.
After that, the fourth section contains two extra lives for you to pick up, but other than that, once you've stepped in further, the section becomes an autoscrolling level where you have to avoid flame shots, Bullet Bills, other enemies, and avoid getting squished between Bill Blasters. Once you've passed all that, you can finally fight Bowser, who rides his Koopa Clown Car. His boss room's layout consists of a flat ground and two red platforms on different sides each, which move up and down to help your attacks reach Bowser. Four Heart items are present, two on the left and two on the right side. An extra life is also present, right at the center of the boss room, on the ground. Bowser attacks by shooting fireballs in a spread shot, but only to the bottom, considering the player is always on the ground while Bowser himself is, most of the time, on air. His starting position is on the right side, then he moves to left, before moving to the center and flying up, going offscreen. He then flies quickly through the middle part of the boss room, going offscreen, then through the ground. After going offscreen once again, he shows up on the right side of the boss room, back to his starting position. To defeat him, you have to shoot Bowser himself enough times.