You may have noticed that every successful attack brings you 1-3 stars. Be sure to press the star button at the bottom of the map — each time you collect 7 stars, you can watch a video to get some extra goodies (lockpicks, gold, potions, etc).

Please note that you can only view a limited number of ads each day. If you run out of videos, the only option to get a Star Master reward is to buy it with gems. Some time after you'll be able to watch more videos.