This is NOT an idle game. Constant attention and app has to stay active. If you close the app during a run you lose all progress. Hundreds of waves and possibly hours of time lost because no save state or background running. The upgrades are minimal to mediocre and scale terribly for what they cost. This looked good at first glance, but too many issues and false advertisement for what you get. There just isn't a payoff to playing it or even keep it running.