Most things are already said, the game is to easy and it doesn't matter what kind of units you are building, just spam the road with anything. It's not clear, what the advantage of the second unit is towards the third, because the speed isn't told in the menu, but it is more expensive:
Variations might be an idea (splash damage, really fast but weak, strong and slow, infectious, ...). Maybe the units could attack the enemies fast (run towards them) and then return slowly to their posts to show the cool down time.
I like the idea of melee fighting tower defense. The animations and the graphic style is very nice, has an atari touch to it.