Nice execution
I like it, it's cool. Man if I coded that it'd run like a sack of crap filled with copious numbers of if statements so I can only say, radcore, I'm jealous of your ability to make something like that run so smoothly.
That said it's the same old thing - I think what would lift the game is to either a. make it more like missile defence and have slower moving projectiles, meteors from outer space and you're firing rockets or something (which has also been done I guess) - or rather have rockets coming at you that can take some serious hits - give the enemy some meat to hit, not just a string but a plane body or something, and make the bullets just bullets with no explosion radius, and able to hit no matter where you click (ie they don't explode where you clicked, just fire off in that direction) - I'd personally get off big time with some of that rapid fire scripting you've done if it was more machine-gunny. Add some background burning cityscape or something, more german planes, make the defence look like WW2 AA guns and you've got yourself a real winner!
That or just do nothing - you seem happy with it, and it's pretty cool so well done :)
If you do take my opinion to heart and make that machine-gunning kind of defence out of this, feel free to PM me I'll be gagging to give it a go!