Only the most successful rulers manage to obtain coveted medals. If you are successful in battle against other players in PvP tournaments, you will get medals as a reward for your rank. You can also receive medals through participating in leveling up Great Buildings. If you are lucky you might find some from Incidents around your city. In the expeditions you also have a chance to win them from the chest after a battle. And lastly from the special buildings you might haven gotten through your gameplay.
Medals can be used to: