Click "battle" to battle. The dots next to the card lvls is buttons to upgrade your cards wich makes your max score higher. The red dot next to the common cards is used to sell common cards. It sells all of them and each one is worth 1 gold. Everytime you win you get a silver chest, and every 2 min a free chest apear.
Credit to Supercell for the original game. When you reach the legendary arena you will fight the person who has the highscore. (not live but max score will be the highscore)