Comets and Craters takes the classic mechanic of open-field Tower Defense games – there is a number of enemy entry points on the left side of the screen and you have to stop them from reaching your base on the right. One of such entry points is reserved for both ground and aerial troops, while two more spit out aerial units. The map itself is the common square grid with a several points where comets explode. And it's actually the leftovers from these explosions that you have to harvest to get the resources to build up your base. The resources themselves come in two varieties – green or blue crystals, with one variety used to build and upgrade towers and the base and the green to repair them.