I agree with Robert's review below, the aliens are becoming impossible to kill unless you spend a fortune in the shop as the rewards received for missions are paltry. It's a sad thing when you receive better rewards for watching adverts (or indeed by playing other games to get gems for this) than you get for playing this game. Also, update 22.50 hasn't fixed the Relief Box glitch, the exiting app on clicking an advert/shut down for no apparent reason we'll have to wait and see....