Bowser is the Key
From the Super Mario Wiki, the Mario encyclopedia
The level begins with many Koopa Troopas and Koopa Paratroopas, along with a Key Door to the right of a Grinder. Mario must defeat Bowser, who is on the top left, to obtain a Key and open the Key Door. In the next room, with two winged Chain Chomps, Mario must defeat Bowser again to get another Key and open another Key Door. In the next segment, Mario must use a Fire Koopa Clown Car to take out a larger Bowser in his own Fire Koopa Clown Car. In the final segment, Mario must take out the Rocky Wrench near the Key Door to enter it. This Key Door will lead him directly to the Giant Gate.
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