You are a strategically placed turret in a valley. You are crucial to keeping the enemy from flanking the city and destroying everything in their path. The enemy has realized this and has sent its entire army after you. Use the mouse to aim and fire to kill all the scouts as they march to try and defeat you, and then the city. At the end of each level you get to upgrade your cannon. Make sure you keep up with the enemy because as the waves go on, they will try harder and harder to wipe you out and take over the city. So do all you can and try to save everyone.