Here in Combat world you can fight with other Dragon city Players from All-Around the World. Combat World has Leagues.
League Reward = 2 + or
For Mobile app –> Reward = 3 + or
To win Each league you have to defeat 7 Players which might be near around your level. Each match will be 3-on-3 battle. If your level is 34 then you will get players to fight in range of 25 to 45 level. You have to choose maximum 3 dragons for battle. You can also battle with 1 or 2 dragons.
The Reward for Each league is 2 Gems(For mobile users its 3 ) & some Exp or Gold. After every league the exp or gold increases but gems will be only 2 for each league.
But you can’t play it continuously as it has time limit for battles. After every 6 Hours you will get 3 chances to fight. If your net connection disturbed or page got disturbed during fight you will lose that match & one chance. If you even click on exit during battle you will also lose the match.
After continuous wins you might face some strong opponents which has higher level dragons than you. It is designed so that you will get motivated for leveling up your Dragons & Playing Dragon city with more interest. After all when opponent is strong you will also become strong.
No one knows how many leagues are there. I think there are infinite no. of leagues. You just have to play & win.
Best of luck.