About "defending the entire screen", there's an issue with that.
This is hard to do if the RNG decides all towers should be on the extreme left side of the screen, bullets take forever to reach their targets on the extreme right of the screen and the knockback means they aren't pushed back, but further away from your bullets, which becomes a problem when they get more health. This pretty much guarantees you can only reliably defend yourself from threats on the left side or the center (which become right side if enemies are pushed by knockback). It's a small niggle, but I think it leaves how much enjoyment you can have to the roll of the dice. Sometimes the game just screws you over for no reason.
