I got two-words... Zombie Mode!!!!!
Fun game. Like the setup and the units. I did become unbeatable after awhile.
Pretty good game, just upgrade one of the units to the max and you are nearly unbeatable so maybe try adding a difficulty setting.
A very good game. But it needs more stuff. Try adding some more levels. And even new types of bonus levels. Additionally, try to make this game more challenging. Also, try adding a level editor or something.
Still, this is quite enjoyable!
Like the game makes you try a little gotta love that in a game