Nice idea, but....
Definitely some room for improvement. After level 5, it gets insanely difficult, as you get very quickly outnumbered. Maybe a way to beat this would be to have a cash reward based on how quick you complete the level, which can then be spent on upgrades like faster production, stronger troops, or maybe like a super weapon that charges up slowly so that once or twice per level you can carpet bomb an area? Anyway, kept me entertained for 20 minutes. Thanks ;^)