Almost every little boy was playing with soldiers, when he was small. Put yourself in a situation, where defense of our base remained only up to you. If enemy units get through our defenses, they will claim important strategic place, that might cost us the whole war, and we certainly can not allow that. Act quickly. Lay out defending units, so that their range cover the most space. You get money for every enemy killed to buy upgrades and new units. Have fun.