I like the idea and it has good gameplay but it is terribly unbalanced. Money comes too slow and it is dependant on the population which decreases with every shot the enemy takes, and no matter how much defences I build they always take a shot which I believe slows or breaks the population's increase. Do a little balance and it will be much better... or PlanetDefender2 maybe?
Either way I still like the art and everything else besides the balance so I give you a 3.5.