World 6-Fortress 1
From the Super Mario Wiki
The level starts with a roller coaster platform, and a few Roto-discs and Podoboos. In the beginning, there is a Super Leaf, which the player needs to collect fast or they lose the roller coaster. Next, there is a door which leads to a room with another Super Leaf, a Starman, and more Roto-discs. Next, there is another door which leads the player to the boss - Boom Boom.