From the Super Mario Wiki, the Mario encyclopedia
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This is a vertical level where the player needs to press Color Switches in order to progress, while dodging the Thwomp and Thwimps. Pressing the Color Switches will create ladders for the player to climb, but also platforms that will allow the enemies to move upward. The player is able to stop the Thwomp from progressing if they press the red Color Switch while it is under the platform, but the Thwimps will always continue. When the player reaches the fourth level, a third Thwimp will join the other two. At the top platform, the player needs to use a Springboard to reach the Key, and then carry it under the three Thwimps to reach the locked door.