This page clarifies some misunderstood rules in the Cities & Knights rules.
Commodities: Commodities are resource cards of cloth, coin and paper.
City Wall: A city wall only gives you right to hold 2 more cards in case of 7. The wall does not protect you from knights, robbers or barbarians in any way.
Upgrade to mighty: You cannot upgrade a knight from strong to mighty (2->3) in the same turn you upgrade from normal to strong (1->2).
No action after activation: You cannot perform a knight action in the same turn after you activate it.
De-activation after action: After a knight action, the knight becomes inactive (actions are: moving, chasing and fighting the barbarians).
Activation after action: You may re-activate your knight in the same turn after you perform a knight action.
Stay activated: A knight stays activated or deactivated after the following events: : upgrade, being chased, being stolen.
Steal mighty knights: Even if you can't build mighty knights yourself, you may steal and keep them anyway.
No cities: If you have no cities, you cannot develop your city any further.
3rd level cloth: You may trade any kind of commodity at a rate of 2:1 with the bank.
3rd level paper: You may pick an element of choice when you have no income (exept when 7 is thrown). So, if you receive no cards because of the robber, you get to pick a card of choice.
No available cities: If you have no cities that are not disabled or metropolis, you cannot develop any commodity past level 3.
Unlimited: You may play as much development cards in one turn as you wish.
Victory point cards: These will be automatically played instantly when you receive one.
Instant play: You may play any development card in the same turn you receive them.
4/5 card maximum: You may never own more than 4 (when it's not your turn) or 5 (when it's your turn) development cards. If you are at your max and you could get another one, you will be asked to throw one away first. When you end your turn you may never own more than 4 cards.
Incorrect texts: On the cards in the real-life game, the texts are often unclear or wrong. In Xplorers these texts are mostly adjusted to the correct rules.
Harbor: If the player you offer a card to has no commodity, there is no trade and you will not lose your card.
Merchant Fleet: You get to pick one type of card to trade 2:1. Trading any type is a mistake made in translation to english.
Trade Master: You actually get to look into the opponents cards and pick 2 of choice.
Bishop: Although the card states you get to pick a card for every town or city at the field, there is a maximum of 1 card per opponent.
Diplomat: You can only build a free street if you remove one of your own streets first. Then you may not place a boat if you took a road and vice versa.
Crane: This card only counts for 1 city development and only 1 card may be used per development.
Deserter: When you steal a strong or mighty knight and you have no left yourself, you may place a knight of lesser strength instead. If you may not build mighty knights yet, you may place one when you steal one.
No cities: When there are no cities, there is no fight. All knight will be de-activated however.
War: When the barbarians land, the number of cities is counted (= barbarian strength) and the number of activated knights is counted (= players strength). The barbarians win when the barbarian strength is larger than the players strength.
Barbarians Lose: The player that contributed the most knight by himself gets a 'defender' card. When more than one players contribute the most together, they all get to pick a development card of choice.
Barbarians Win: Of all the players that CAN lose a city (so players without cities or with all metropolis do not count), the one(s) that contributed the least have to pick a city of their own to be replaced with a town.
Lose city: If you lose one of your cities but you have no town available, your city is temporarely downgraded to town. You must restore this city-town to a full city again (normal city price) before you can build other new cities.