Because of the new gameplay elements introduced in Cities and Knights, the game is typically played to 13 victory points, as opposed to 10 as in the Settlers of Catan base game.
The commodities are paper (which comes from forest terrain), coin (which comes from mountain terrain), and cloth (which comes from pasture terrain).
When combining Cities and Knights with Seafarers, the rules are ambiguous with regards to whether commodities are collected along with normal resources when collecting from a Gold River tile, as well as whether or not commodities can be collected directly from Gold River tiles.
A city on grain or clay gives 2 of each, as in the original Settlers. While wool, ore and wood yield cloth, coin, and paper, respectively, grain and clay act as fuel in special functions: grain activates knights, and clay can be used to build city walls.
The first player with an improvement at the fourth level can claim any of their cities as a metropolis, worth an additional two victory points. Each type of improvement has only one associated metropolis, and no city can be a metropolis of two different types (because of this, a player with only metropolises and settlements may not build improvements beyond the third level). If a player is the first to build an improvement to the final level (out-building the current holder of the metropolis), they may take the metropolis from its current holder.
Knights are placed on the board in a similar manner to settlements, and can be used to block opposing roads, active or not. However, knights must be activated in order to perform other functions, which immediately deactivates the knight. Knights cannot perform actions on the same turn they are activated, but can be reactivated on the same turn as performing an action. These actions include:
If a knight is promoted or forced to retreat, its active status does not change.
The barbarians are represented by a ship, which is positioned on a track in special "double-hex", representing the distance between the ship and Catan (ie. the board). Each time the event die shows a black ship, the barbarian ship takes one step closer to Catan. When the barbarians arrive at Catan, a special phase is immediately performed before all other actions (including collecting resources). In this special phase, the barbarians' attack strength, corresponding to the combined number of cities and metropolises held by all players, is compared to Catan's defense strength, corresponding to the combined levels of all activated knights in play.
Should the barbarian attack be successful (that is, the attack strength be greater than the defense strength), the player who contributed the least amount of defensive points is forced to reduce a city (not a metropolis) down to a settlement. If there is a tie among this distinction, then all tied players must (if possible) reduce a city to a settlement. If the lowest contibutor(s) only have metropolises, then the next-lowest contributor(s) must reduce a city to a settlement. It may be the case where players tie for the lowest contribution, with some of the players lacking cities to reduce (in which case the players without cities is exempt), or the case where all players are reduced to settlements and metropolises, in which case nothing happens.
There may also be the case where reducing a city would leave a player with six settlements (a player is only given five settlements), in which case the city token is turned on its side until another city is built.
Should Catan prevail, the player who contributes the most to Catan's defense receives a special Defender of Catan card, worth an extra victory point. Regardless of the outcome, all knights are immediately deactivated, and the barbarian ship returns to its starting point on the track.
In the event of a tie among the greatest contributors of knights, none of the tied players earn a Defender of Catan card. Instead, each of the tied players would draw a progress card (explained below) of the type of their choosing. Similarly, if there are no Defender of Catan cards remaining, a progress card is drawn instead.
As the likelihood of having the barbarian move closer to Catan is very likely, under Cities and Knights the robber (and with Seafarers, the pirate) does not move until the first barbarian attack, nor can a knight move the robber before that point.
Examples where cities are lost:
1) Player A has 3 cities and 2 active knights. Player B has 1 city and 2 active knights. Player C has 2 cities and 1 active knight. When the barbarians attack, player C will lose one of his cities, because the attack strength (6 cities) is greater than all knights combined (5 knights).
2) Player A has 3 cities and 2 active knights. Player B has 1 city, which is a metropolis, and no active knights. Player C has 2 cities and 3 active knight. Player B's city is a metropolis, and metropolises cannot be destroyed by the barbarians, so Player A loses a city because they have the next fewest number of active knights.
3) Player A has 2 cities and 2 active knights. Player B has 3 cities and 2 active knights. Player C has 2 cities and 2 active knights. All players will lose a city, because they all tie in the number of knights activated, and the barbarian attack strength (7 cities) is greater than number of active knights (6 knights).
4) Player A has 3 cities and 4 active knights. Player B has 2 cities, which both are metropolises, and 1 active knight. Player C has 1 city, which is a metropolis, and no active knights. First in line to lose a city is player C, but because his city is a metropolis we need to look at the person next in line. This would be player B, but the same applies for him: he has activated only 1 knight, but both of his cities are metropolises. This leaves player A left to lose a city.
Progress cards are organized into three categories, corresponding to the three types of improvements. Yellow progress cards aid in commercial development, green progress cards aid in technological advancements, and blue progress cards allow for political moves. When a castle appears on the event die, progress cards of the corresponding type may be drawn depending on the value of the red die. Higher levels of city improvements increase the chance that progress cards will be drawn, with the highest level of city improvement allowing progress cards to be drawn regardless of the value on the red die.
Progress cards, unlike the development cards they replace, can be played on the turn that they are drawn, and more than one progress card can be played per turn. With the exception of two types of progress cards, however, they can only be played after the dice are rolled. Progress cards granting victory points are played immediately (without regards to whose turn it is), while the Alchemist progress card, which allows a player to predetermine the roll of the white and red dice, necessitates the card being played instead of rolling the numerical dice. (The event die is still rolled as normal.)
Players are allowed to keep four progress cards (five in a five to six player game), and any additional ones must be discarded on the spot (unless the 5th card is a victory point, which is played immediately and the original progress cards remain).
If the barbarians pillage your city, then the city wall is also destroyed and the wall is removed from the board.
The merchant can only be deployed through the use of a Merchant progress card (of which there are six), on a land hex near a city or a settlement. The player with the control of the merchant can trade the resource (not commodity) of that type at a two-to-one rate, as if the player had a control of a corresponding two-to-one harbor.
The player with the control of the merchant also earns a victory point. Both the victory point and the trade privilege are lost if another player takes control of the merchant.