This soldier has to guard an army base. Place them strategically, and let him do his work: he automatically aims his gun at the enemy soldiers passing by. You can also build defense towers. You can upgrade the towers you have or sell them if you think it's necessary. You can also call in auxiliaries (at the bottom of the window): they walk in the opposite direction and shoot at all the enemy soldiers they come across. Don't let the enemy reach the exit!
Control your character with the mouse cursor: he'll walk there, where you click. If you want to buy a tower, you click on a spot, and then you choose a tower in the menu. If you click on it, the tower is built.