A great game, but lacking in finesse
While this is, as I said, a great game, which is really well made and thought out, it lacks the final touches that make a game truly awesome.
Firstly, the last assault rifle, for 4000xp was easily the best gun, much better than the 9000xp minigun. Once obtained, the game is easy to the point of boredom. The game was very easy before that, but after that gun was obtained, it was simply a matter of standing next to the base, one hand scratching my head, the other moving the mouse, holding down the left mouse button.
The engine for this game could have been put to better use perhaps in an rpg-type game or an adventure game...
Basically, you really needed to tweak the hp of enemies/damage of the guns, reload times, enemies speeds and cost of guns to make this game more challenging.
But I did find it very entertaining for half an hour, which is pretty good for a flash game. Great job! :)