Even suffering through ads and not buying the game, up until the final boss battle, I found this game challenging and fun to try and get through.  Level design was really well done and found myself truly challenged.  However I found the final boss battle impossible to tolerate since you die a lot and between each try you are forced to watch a 30 second ad.  So I found quickly that you'd spend 15 or 20 seconds trying to figure out a way to beat the boss, ultimately die, and suffer through a 30 second commercial over, and over, and over, and over, and over.  Finally, I could not take it any further and uninstalled the game without ever beating the boss.  Up to that point, I very much do recommend this game.  But BUY THE AD FREE VERSION if you want to beat the end boss.