Frontier Guard starts you off with a row of circles at the bottom of your screen in red, green, and blue. Click on the different circles and they will shoot same-colored bullets towards the circles coming at you from the sky. The circles do not always shoot. Some need more time to recharge than others. Also, don't let any of those circles get past your defense or it will take out the circle it touches, and before long you're left with nothing and it's game over! See how long you can last without losing your defenses in Frontier Guard!