I don’t know when this was release because the App Store doesn’t say, but there’s nothing to the initial mechanic, but you CAN play offline if you start the game first then turn off your WiFi and cellular. Boom, no ads. You won’t play long because even upgrading costs exponentially more for minimal benefits, and no mechanics upgrade. Dig more graves, carry them one at a time, fight them.
Also? The shooting only works like 60% of the time. A lot of the shots just do no damage at all and disappear making it hard to kill enemy’s when they group up. Not worth the 30 minutes to get the main part of the game.