The only gun and hundreds of alien ships that are eager to land on the surface of the planet and start a real invasion is the game Alien Defense. Your task is to hold the defense by all available means and forces. They are few, but the rest is compensated by your dexterity and quick reaction. Even one weapon can inflict a substantial level on a large enemy. Turn the barrel and shoot first of all at those who flew closer to be sure to destroy. Make sure that they do not reach the surface, this will be a defeat.