You might notice that on some levels you’ve earned three stars for, a fourth star will become available to earn. It will look like this:

Play these levels again and earn a higher score to unlock the Crown Badges. They are quite difficult to earn and provide an extra challenge. When you win a level with a crown badge by accumulating a high enough score, the badge will be added.

You can check the number of Crown Badges available in specific regions by tapping the ‘Map’ button on the left side of the game screen. Good luck!