World 6-Fortress 2
From the Super Mario Wiki
This level is a very simple level made mostly from Ice Blocks. It starts with a sliding Thwomp, a Boo, and a ? Block with a Super Leaf. Next, there are three more sliding Thwomps before a second ? Block with another Super Leaf. Next, there a few Roto-discs, three more sliding Thwomps, another Boo, and a Warp Door which takes Mario to the boss - Boom Boom.