Medals can be won in player versus player (PvP) tournaments. Each technological age has corresponding towers in the world map. Unlock the province with the tower to see the tournament statistics. Their main purpose is to unlock city extensions.
To win medals, you have to fight and defeat your neighbors to gain points. You can also attack players in the neighbor menu.
Every Sunday, the tournament closes and winners obtain their medals, the amount of which depends on the age the tower corresponds to.
Medals could also be traded with Leonardo Da Vinci for blueprints during the Feb 2013 event.
Medals can also be earned by helping others develop the Great Buildings.
Medal Scoring in PvP Tournament Tower
The number of medals needed to unlock a medal expansion increases with each expansion that has already been purchased. The first one costs 10 medals, the second costs 50 medals, and so on. The cost of each medal expansion can be viewed here.