It has been a VERY long time since I have played an excellent defense style shooter such as this one. So many of the defense shooters that I've played revolve around the Earth or the people of the Earth defending something against strange, hostile alien attackers. In this one YOU play the aliens, trying to stop the earthlings from invading your planet and stealing your energy source.
The game is very well put together, the upgrades are nice, and, unlike a lot of these games, the prices do not go from very low to exorbitantly high after you buy one upgrade; I was able to upgrade something at least once after every round. The rotation of enemies is nice as well, although you could put a little more variation into them.
All in all, this is an superb game. I know that a few reviewers said that you should make a sequel, and I think you should. You could add more upgrades, more power ups, more enemies, the list is endless as to what you can do. I think if you stick with what you have premise of it I you would have an excellent game to build an remarkable sequel off of.