Ok, I got to level 14. Then it got a bit crazy. The key to success is mathematics. Speed increases are always worthwhile due to the way the firing delay is calculated (but should be balanced with adding a few archers and maybe upgrading the damage once or twice). Once you've maxed out your firing speed, it's choosing between buying a new archer and upgrading the damage which is the trick. It's a question of value: The value of an archer is (current damage / archer price). The value of a damage upgrade is (5 * number of archers / upgrade price). Whichever has the best value is the one you should buy. (The 5 in the latter calculation is because a damage upgrade increases damage by 5). Still no death star in sight...

Really good game. The thrust feels a little bit unresponsive though, and the ship appears to lurch when the rocket flame appears/disappears - I guess the images are different sizes? You could consider using a separate actor for the flame (though you'd need to position it well).