I just finished the game and it's a pretty sweet game. Keep then comin' .
My only complaints were with the towers, as other people have already said. If you could buy a level up, then you could have had many more strategies. An obvious solution would be to build your defense from the top down, but I assume that a lot of people missed this.
As for the towers themselves:
I really liked mudman, but it was too expensive and evolved rather slowly.
I disargee with LordThanatos, dragon was clearly superior to the demon, especially when leveled up, dragon has three different attacks when leveled up, and all of them deal damage: pushback, additional damage, and frost breath, which freezes the enemies solid for a couple of seconds. In addition to this, the skeleton needs to evolve 3 times in order to increase his attack power, the dragon gets it after the first level up. I suggest bulding a slime first, than a dragon and after that the artillery. :D
I also faced a dilemma with plants, when they evolve for the second time, they lose half of their range, which I really disliked, becouse my strategies were mostly with choke points.