Very nice but.........
The game is nice, but after a while it gets boring. Instead of every level increasing the amount of troops, rather, you should add variation.You should add harder enemies, with different animations and models. Also, you should add the ability to add turrets, or buy other people to help (you know, like mercenaries).
The music was very nice, it wasn't the crappy heavy metal music, but soft music that catches the feeling of the apocalypse.
Also, being able to buy different guns would nice, and being able to upgrade every one individually.
I know it's a lot to ask for, but they're just some suggestions.