Well...
I agree with what some of the other people have said that it is a good game that does need some tweaking. Not that it wasn't fun to play, but it does need some work. Here are a few of the issues I noticed:
1. Weapons and upgrades, I noticed someone else had said this and I wanted to elaborate. Some of the weapons are rather useless. The first two machine guns rather suck, firing normal with a fully upgraded pistol does more damage per second than a fully upgraded Uzi. Considering the cost difference those guns and upgrades should ultimately do much more damage than the pistol, certainly not less.
2. Along the lines of weapons and upgrades there is not enough money in the game. In fact there is not nearly enough money in the average game to fully upgrade the minigun. Though i am aware that your enemies do spawn randomly it would take an exceptional game to produce enough money to get that gun fully upgraded (I was at least 60k off when the game ended, though my guess is there was another damage level after the 40k level and probably another clip size after the 20k level which puts me at more like 150k off). You should be able to have several weapons fully upgraded by the end of the game. A good way to deal with this would be to make the game longer (i.e. add more levels 50 sounds good), make enemies worth more, or reduce the cost of certain upgrades. You could also add a different mode, allow the game to be re-started with everything you ended the game with before. Though I am fine with there not being enough money to purchase and upgrade every weapon there should always be enough so that a player can fully upgrade several weapons (50% is a good number) And that is not even factoring in the potential upgrades to the fortress, riflemen, and engineers.
3. The quality button doesn't seem to do anything.
4. I like the sounds but don't care for the music, it would be nice to be able to turn them off sepperately.
5. The game ends rather abruptly, you may want to think about changing that.
6. Bosses are good, even if it is just one boss at the end of the game.
7. There aren't generally any breaks in the waves, the reload button isn't nessesary.
8. I agree with what others have said that the igloo should look different when it has been upgraded.
9. As was previously mentioned reloading the game you loose previous upgrades to weapons.
10. A sandbox mode can be nice as well.
11. Regardless of what the weapons stats are it would be nice to be able to check them out before shelling the money out to buy them. I felt totally jipped the first time I played and bought the machine guns.
12. Perhaps when a person dies durring a level you could kick them back to the prior shop screen rather than just killing off the enemies and garnishing the score.
Again very good game, but lots of room for improvement as well. i hope you either put out an updated (corrected) version, or a sequal that includes peoples suggestions. Good luck and keep up the good work.