Any player who has attained Level 10 may create a League. In order to do so, go to the League Hall and open the "Create a League" tab. You will have to come up with a unique name and tag for you League, as well as design a banner and pay the League creation fee, which is 1,000 Sapphires. The League tag is 3 letters that indicate which League a Beacon or Castle belongs to on the Map. After doing so, you will become the Marshal of your League, and will be able to accept other players' requests to join.