The player will first encounter a Dry Bones that throws bones at Mario, and two large spiked pillars that come crushing down at full speed. There are two trampolines for the player to get on the next block. After the block are Grinders, with one little space provided for the player to avoid them. Podoboos jump up and down in the next segment, and after that is a combination of Grinders and the spiked pillars. If the player manages to safely make it out of that part, there are small rock blocks and Grinders along the trail. There is a Yellow ! Block, and shorter spiked pillars that come up and down even faster, so that the ground keeps shaking. Right after is a midpoint and the door to the next section.
The next section involves moving blocks, Lil' Sparkys and Hotheads. The player must be careful in the middle section of the second part; if the player makes a mistake, Mario may be crushed to death. At the end of the second part is the boss door that leads to Wendy O. Koopa, and a Green ! Block in front of it.
The fight against Wendy O. Koopa is similar to that of Lemmy Koopa in Castle #3, except the tubes are flatly leveled, and that there are two Podoboos that move diagonally and hit the walls of the level. When Wendy is defeated, Mario paints over the castle and rescues an egg. The cutscene text reads: "Wendy O. Koopa in Castle #6 has sung her last song. Mario must meet the challenge before him. There is a sunken ship that appears to be a gateway to the valley of Bowser." Mario then moves on to the Sunken Ghost Ship.