Cool idea.
The levels (like almost everyone's games mine included) need a bit tightening up.
It actually took me a while to figure out the theme, since I never needed to use the blocks to defend against enemy fire. I could just turn them off and use good angles to avoid them.
Or maybe it was because the pre-existing block worms didn't respawn when I retried a level. I noticed sometimes I would swap over, worms would appear and ambush me, but when I restarted the level, they weren't there.