In order for the game to be a reasonable difficulty you have to constantly upgrade your units which costs coins. You barely get any coins from missions even while watching 1-2 ads after every single mission. Half the units can also be upgraded with real money, and a lot of it at that. This game is a cool concept but way too big of a money grab. Not to mention the AI just sucks. You can have 5 units out and zombies will still just walk past you. Your units won't turn around to shoot zombies.