For some reason, I started out thinking this was a stupid game... But I looked at the clock and I've been playing it for half an hour now. lol Needs a sequel!
Also, as a couple other people commented, you need to add an upgrade for starting money. Some of the later levels, it's nearly impossible to get them perfect because you start with only enough to build one turret.
good game, but it needs a bit of tweaking. The mortar should be long range with high damage and slow rate of fire. It says slow rate already, but it shoots like a machine gun. lol Not quite long enough between waves. Planes don't cost any lives when they get through.