A pillar of the human race is the new Space Station Xorbe in Earth's Orbit. This space station lived in peace until the aliens arrived. Now those same aliens want to use Xorbe to stage an attack on earth. Now it is up to you to protect Xorbe, and therefore the world. Place your air defenses first, and then place all the other towers you need to find off the aliens. If the aliens touch down on the base, then that square is completely unusable for any other means. So upgrade your towers as soon as possible, because if a second alien hits the same square, the aliens can start attacking YOU.