Great game
Space Defenders is a classic game and it is great that you are trying to put a new spin on it. It reminds me of Weebl's "Inverse Shoot 'Em Up". It's a game like this where you fire at the guy who would normally fire against the enemy as you are the enemy. The best part is that the atmosphere is really good and quite chilling. The music was really cool and you were getting more into the idea of making something dark. I guess that if the aliens win, that would be pretty dark.
After awhile, there are so many enemies you have to simply dodge the bullets and not care about attacking. I mean, with so much movement you're bound to hit most of your targets. The worst things were the homing missiles or bullets or whatever. It was also cool to be able to go higher or lower, but not all the way low. This is a great tribute to a classic game.