Simple and fun at first. Challenge level increases over time as expected. The game has a built in ad watching for rewards system like most free to play games these days. Gives you increased rewards so you can upgrade your ship a bit more. Problem is for the past week the ad system doesn't work. Also the ship upgrade levels is ridiculous. The max upgrade level is 27000+ and you HAVE to upgrade one at a time. So to hit max level you have to tap that upgrade button 27000+ times!?!?