Zombies are kinda overpowered....
Playing as the Orbelugs, I have to say that the zombies are way too powerful once you've maxed them out. They shouldn't be able to take out the land bases in one hit. Other than that, the game is pretty entertaining.
My only suggestions would be to add more differences between the factions (each faction is pretty much the same until you reach the end of the game) and to increase the usefulness of certain things, like the defense towers and special weapons. With some more features to stave off redundancy and a decrease in the number of glitches, this game would be a perfect 10.
Also, how cool would a sequel including factions from each planet be?