In total there are twenty missions that you can play and adjust the game mode for. Besides the story, you can play with fixed resources, limited number of towers, or upgrades for them. There are two grinder modes that have one-hundred waves of aliens; super grinder mode is more challenging with multiple alien species to contend with. For those looking for a challenge you can damage the aliens manually in power outage or focal point. In those modes you have to control the towers or deal the damage yourself.