Destroying the right orb will open the path between the start location and the bottom room.

Destroying the middle orb will open the path between the bottom room and the middle room.

Destroying the bottom orb will open the path between the bottom room and the left room and spawn the 2nd wave.

Destroying the left orb will open the path between the middle room and the top room and spawn the 3rd wave.

Destroying all 4 orbs will open the path between the start location and the middle room.