I've never felt sorry for enemies in TD games, but these towers are OP and they don't stand a chance. You should be able to reset stats (as in most games), but overall a good game. 4/5 - really enjoyed it. ;)
Uncollected bubbles should remain if you should accidentally leave the area they're in. And I agree a sample of new gun firing should be added. Overall, good game - 4/5.