Fun little game.
I do believe I've played something of the sort before, an old game on some game system about defending something of the sort from alien invaders... Was that an inspiration? Either way, this is a good game.
I'd give it a perfect five and ten, in fact, except for one small thing. You see, it got a bit too hard after the giant aircraft passed over the first time. You probably should have had a sort of "difficulty select" thing, with the default speed and frequency of aircraft is "Normal" mode. Maybe it's just that my mouse is acting retarded, but still, it was a pretty hard game.
5/5, 9/10.