When towers aren't enough, you can call in air support such as airstrikes and tactical drones to clear the battlefield.
//play.google.com/store/apps/details?id=com.refineryproductions_2112td&hl=ln">Google Play and the App Store : 2112TD-Tower-Defense-Survival.rar
You can deploy and upgrade various turrets, including machine guns, flamethrowers, artillery, and plasma turrets. When towers aren't enough, you can call in