Oh my gosh, this is one of the best Unity platforming games I have ever played. If I were a programmer, I would totally make this into a multiplayer shooter.
WOW... All I can say is that this game is very awesome! There are a few things you might want to work out:
1. PLEASE make the projectiles a different color so that you can spot them easily.
2. When the player finishes an objective, that single objective should be replaced, and even awarded some money.
3. It would be nice that instead of having upgrades to change guns, you should add a leveling system to each of the guns to make the leveling system more versatile.
4. I haven't gotten that far in the game, but I noticed that there aren't that many turret options. If you could provide more options and integrate #3 into this, that would be AWESOME!
Thanks for reading this! It would also be cool if you could port it as an iPod app.