As Cities collect Commodity cards, they are spent to upgrade the City Calendar. The City Calendar has three city improvements, one for each Cities and Knights Commodity, Cloth, Coin and Paper. These improvements are called Trade, Politics, and Science, respectively. Each upgrade purchased in a path increases the potential to draw a Progress Card of the corresponding improvement. When the red die is rolled on a 1 or a 2 and the event die is rolled to a non-black color, players owning the first level all draw one Progress Card of that color.
At the third level in a city improvement, a special ability is unlocked.
The special ability for Trade is called the Merchants Guild, which allows the player to trade Commodities at a 2 to 1 rate during their turn.
The special ability for Politics is called Mighty Knights, which allows the player to upgrade their Strong Knights to become Mighty Knights, who have a strength able to protect three cities, and can be used to displace opponents' Strong Knights.
The special ability for Science is called the Aqueduct, which provides a free resource card whenever the roll is not a 7 and the player does not otherwise collect any resources. This works even when a player has a number but the robber is on that specific hex.
The first player to reach the fourth level in a city improvement is given a yellow Metropolis token to place overtop one of their cities. This city cannot be downgraded when the Barbarian Ship Attacks, but it still counts toward the total Knight power required to defend Catan. If another player is able to reach the fifth and final level in the improvement before the owner of that Metropolis, they may steal that Metropolis. A player may own up to three Metropolises, one for each city improvement. Each Metropolis is worth an additional two victory points to the player who owns it.