Defend different cities against waves of aliens! Start off with a pistol and unlock new guns by completing levels. You will also receive gold and up to three stars for completing each level. Spend your stars in the Tech Tree to upgrade your weapons and use your gold to purchase power-ups (in the end of a level) that will help you a lot. Do you have what it takes to fend off the alien invasion?
Click to shoot, use Space bar to aim down your gun sight. If you buy power-ups before you start a round, you should click their icon in the top left of your screen to use them.