Press the mouse on the left side red dot, and draw a line from the left side red dot to the right side red dot. Press the play button. Drag the gun with your mouse and place it above the creep's path. The gun shall not touch the path. Please see the figure below. Press the mouse to fire the gun.
Some of the creeps are immune to the bullets, and they cannot be destroyed. If you kill 20 or more creeps in a level you go to next level.
If the number of creeps in the fort exceeds a certain number the game ends. Maximum number of creeps allowed: level-1: 40, level-2: 35 and so on.