On one of the planets, which was colonized by earthlings several bases where people lived, worked and studied the planet's surface. But one day, not far from one of the aliens landed. First firing occurred with one of the search party and those Earthlings retreated to their base. You in the game Alien Defense will have command of the Defense Forces. On the road that will lead to the base will be promoted by aliens. You need to build a system of fortifications along it. After all, in the towers will be located gun that can destroy enemies. At the bottom of the panel will be located their icons. You need to select the desired and move on the playing field. Thus, you will create a protective and defensive line.