i like how the first tower can still be useful even during the last 10 waves. just pop up a bunch of them near some slow towers (and keep some lvl1 towers around to intercept the healing of healer bugs).
the thing that makes this game so hard is the cops. they are always in the way for me and ones i hit one at a got speed then i just get slown down and have to use up all of my rocket jumps on getting myself over the gummies. but still, its a god game.