As others have stated, this is clear pay to win advertising machine. Want to restart a level? Que advertisment. Beat a level? A fresh ad is your reward. The gameplay is actually pretty slick on the first batch of levels but becomes near impossible very quickly unless you are willing to buy a hero/upgraded tower, spend gems on power ups, etc. Do not put on a device that a child has access to because the "deals" that they're pushing are quite expensive and a few seconds could cost hundreds.