A metropolis is a type of building in Catan: Cities & Knights.
Basic function Edit
When a player achieves the fourth level on the City Calendar, that player may designate one of their cities a metropolis, so long as no other player has already claimed the metropolis for that City Calendar column.
Upon designation, place the metropolis piece (a wooden yellow arch in the Mayfair edition) straddling your city. Your city is now a metropolis. It receives the same resources and commodities as a city, but is now worth 4 victory points. It also cannot be razed by the Barbarian fleet.
A metropolis may be downgraded if another player reaches the fifth level on the City Calendar for that metropolis before you. If your city is downgraded, the arch goes to the new metropolis and the city is once again susceptible to the Barbarian fleet. It also loses its bonus victory points.
A player may, hypothetically, obtain all three metropoli for each City Calendar column. Some fan expansions may add additional metropoli or change the rules for obtaining them.