Leagues are collections of opposing Alliances who are grouped together based on the amount of Victory Points they have.
Victory Points (VP) are gained from battles when Raiding or Capturing other players' Cities or Lands. Certain Lands (lakes and forests) generate VP by themselves.
When each season ends everyone's VP is reset to zero and each player gets a reward based on how well their Alliance performed this season. The better an Alliance performs in its League, the better the rewards each member of that Alliance receives. In addition, the best-performing Alliances are promoted to a higher League, while the worst-performing Alliance are relegated to a lower one.