It's not free!! The game is smooth, amazing, and wonderful, but it's not free, to continue your campaign you should pay. You're allowed to play the first level, or play a single game (which is boring...) by watching an ad. If developers placed ads between the game would be much better for everyone, and the one who is annoyed from ads can pay to remove them because by forcing player either paying or quiting will generate a lot of haters and low ratings which won't make this game recommended.