D06.3 is a secret level which is unlocked by completing D06.2.
Just like the previous level, D06.3 is a Tower Defense level. This one introduces the player to the anti-turret enemies. They behave the same as big enemies, but they can disable nearby turrets. You also have a horizontal gun.
In order to win, you will need to both switch the turret types correctly and destroy the anti-turret enemies quickly with the horizontal gun.