From the Super Mario Wiki, the Mario encyclopedia
There are two parts to the castle. The player begins in an area in the castle where the Koopalings are found, acting as they did in World 6-. Two ? Blocks are found, along with a rope above a Grinder and a pit of lava. The rope must be climbed past a Grinder jumping off a track, and two sets of Donut Blocks are found at the end. More Donut Blocks, a Grinder, and some fences. A platform with a Dry Bones is found, along with more Grinders and Donut Blocks. A rope above a platform with spikes on it is found, along with more fences. A Warp Door leading to an area with a Checkpoint Flag and more Grinders is found.
The player then enters an area with a Snake Block and the Koopalings. Grinders are encountered as Mario rides the Snake Block, which occasionally goes into the lava. The boss door is found at the end of the area, and leads to a second Checkpoint Flag and three ? Blocks. The player must then battle Dry Bowser. This battle is the same as the Bowser's Castle level. On the first phase, he breathes out several blue Fireballs before jumping, allowing the player to hit the ! Switch. On the second phase, the Koopalings make Dry Bowser large. The player must jump on some platforms to reach the giant ! Switch and defeat him.