very fun
But too short! Not enough variety of upgrades and ennemies!
Upgrades should be more expensive
Upgrades that cost a lot, like the satellite or whatever, if you want to limit the number of times a playe can use that "ressource" add a cooldown time for those special weapons.
Add more ennemies that are a chalenge! Faster, deadlier and more accurate ennemies! (Not too much, but a few are always fun)