It took a few attempts, but I managed a run that reached wave 13! The balancing of the game is probably exactly where you want it - I kept thinking 'my defense will last forever!' and then the next wave would always prove me wrong.
Art is good, love the concept, wish there was more! More varieties of trees, a way to de-toxify the area, more enemies... I think this kind of tower defense has a lot of potential, and I'm surprised I haven't seen an 'area control' style of tower defense game before already. It works so well!