Did you notice the Insubstantial Blocks that can be found in various levels? There is a switch in Level 1-5 to turn those blocks into solid ones. The entrance is blocked by three doors and the Iris puzzle needs to be solved before the room can be entered. After jumping on the pressuse plate the level still needs to be finished in order to enabled the solid blocks. This action cannot be undone, but it has no negative side effects either.


These blocks and the switch to activate them are a reference to Super Mario World where 4 switch palaces can be found that activate different kinds of blocks.