From the Super Mario Wiki
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The player starts out on a platform, and must drop down onto another platform that contains a + Clock. Dropping down two more platforms leads to some rotating gear platforms and a Rectangular Coin Block that contains two Coins and a Statue Leaf. The player then proceeds to a platform that is propelled upwards by lava, and then run across a platform where they must avoid three Lava Bubbles. This leads to a platform where two Hammer Bros. reside; beyond that is a drop down to a lower platform with a Fire Bar, and finally, a platform that contains several Thwomps.
After this is a long narrow walkway that leads to the Dry Bowser fight, where the player must proceed to the switch whilst avoiding Dry Bowser's Fireballs. After defeating Dry Bowser the door to the Fortress opens, leading the player to unlock Luigi, then there's a Warp Box that will lead to the Goal Pole.