I found the game pretty fun, my strategy for easy:
1) Upgrade main gun rate of fire and damage once or twice
2) upgrade repair bay to 2 drones and faster repair (or more repair)
3) add another ship bay and upgrade both bays half way, but not build rate
4) upgrade main gun and repair bay more
5) add two more ship bays and upgrade them evenly to the first
6) fully upgrade all 4 bays, main gun, and repair bays
7) I never needed any turrets and beat the game w/ ease
What the hell? I was gonna give it a good rating, but after beating 80 or so levels yesterday and coming back and having 0 levels completed this gets a big fat 0.
This is the first game I have bothered to rate, only because it is so horrible. It doesn't register keystrokes if you have a WPM rate faster then that of a snail as well as the countless glitches. As I write this comment my ninja is standing up, with "tap up quickly" still being displayed, totally preventing me from continuing playing.