From the Super Mario Wiki
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The player has to first climb the ropes and press the blue Color Switch, while avoiding the Firebirds. Pressing the Color Switch will open the wall down below and the player can continue onward, avoiding a Kurasshā. Next, the player has to climb a ladder while avoiding a Katakata Kaen Heihō at the top, and then tossing the Katakata Kaen Heihō onto the ledge to the right so the player can reach the ropes. Continuing left, the player can grab the Key and use a Ninji to reach the locked door.