In order to climb up the Leagues you will have to complete Divisions by collecting a certain amount of Gold Bars.
In order to do so you need to convert your Coins that you won in duels into Gold Bars.
Four types of Forges: bronze, silver, golden and ruby - will help you to convert your Coins into Gold Bars.
Different types of Forges are capable of converting different amounts of Coins. Only one Forge can work at a time.