I don't object to adverts but they make this game crash. It has been a problem for a long time. Sometimes it is so bad it is impossible to complete challenges. Other reviews complain that PvP mode is rigged, I don't think that is the case. You do not have to pay either, you can steadily upgrade playing for free. EDIT: The developer has repeatedly advised me to update or clear cache, neither helps. Only when the problematic ad goes out of rotation does it end, until the next offending ad...