It has some great promisses, but i encountered some weird bug where my graphics after lvl 2 went into this simplyfied mod (see prntscrn). I dont know if its intentionall. Also didnt know what some upgrades do, nothing that simple description wouldnt fix. And the fire rate of the machine gun is too slow (or the atacks of enemies are too fast) for my taste, but if its for sake of balance, be it.
Despite some problems I see great simple game in front of me! Keep up the work!
(Will wait with rating after you reply :) )