Avoid the rocks!
From the Super Mario Wiki, the Mario encyclopedia
The eponymous rocks of this level emerge from large holes in the walls, fall down and shatter on the ground. There are two different sizes of rocks: Small ones that can be picked up and thrown like enemies to get rid of Enemy Blocks and big ones that crush Wario underneath them, forcing him into his flat form. The flat ability is of little use in this level, so the player should try to not get crushed.
Hidden Treasure: Star Gem
The hidden treasure of this level, the Star Gem, is located in a secret room in the area with the platforms, the Togemen and the D.D.s. Here a platform with a single coin, connecting to the wall on the right can be found. The player can make Wario smash through the wall and discover a secret passage to said room here.
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