The goal is to kill the red dots moving around the map (the "creeps") by building gun turrets which shoot the creeps. To do that, click on a gun turret box at the bottom of the screen. For instance, click the red and black gun at the lower left part of the screen. A copy of the gun will attach to your mouse which you can then place in the map by clicking on a map tile. The yellow area around each gun indicates how far and where it can shoot. Each gun has certain characteristics so you can't use them all the same. The guns can only shoot at creeps inside their fields of attack. Some turrets do splash damage which can kill your own turrets so placement is very important. Other guns are 'shoot through' guns which fire like a laser all the way through the creeps in a line, doing damage to them all at once. These guns can also kill your own turrets if you're not careful. Finally, most of the guns can be upgraded and it's not possible to beat the map without doing so. The upgrades cost money but make the guns more effective. You can also click on the different map tiles to see how they're different from each other. For instance, black wall tiles block your towers' sight and water tiles can't be build upon.