Cities and Knights is the best Catan expansion made. It adds new rules that create a different style of gameplay, but don’t change the main idea of the game. The new rules are simple enough that you can read them and then play without someone else explaining them to you, but complex enough that it gives you a good reason to purchase it.
There are now three new “commodities”, paper, coin, and cloth, which come from the forest terrain, mountain terrain, and pasture terrain respectively. These commodities are used to upgrade your city, which means that 1) you can get more progress cards (the replacement for development cards, which I will explain more later) and 2) give you special benefits depending on how much you progress in each. Paper by far has the best benefit, giving you extra resources of your choice whenever a number besides a 7 is rolled and you don’t collect any resources from that number. If you do enough city upgrades in a certain path (cloth gives you trade, coin gives you politics, and paper gives you science) you get to build a metropolis, which are worth 4 victory points, rather than 2.
The next change is the progress cards, which replace development cards. There is no longer a knight card, because there are physical knights, and there are many new cards, including saboteur, which causes people with the same number of victory points as you or more to have to discard half their hand, mining, which gives you two ore for each mountain terrain your next to, and more.
The next addition is city walls. These increase the number of cards you can safely hold in your hand by 2 each, up to a maximum of 6.
The final and most important addition was the knights and barbarian attack. Every turn you roll an extra die, which signifies a special thing on each side. Half of the sides represent different city improvements, while the other half represent the barbarians sailing closer to Catan, and eventually attacking it. Players build knights and upgrade then to increase their strength, protecting then from the barbarians. A number of barbarians attack equal to the number of cities on the board, and depending on the total strength of each player, various things can happen. If there is an equal number or more of knights to cities, then Catan is safe, and the player with the most strength gains a victory point. In the case of a tie, players who tied get to choose which city improvement path they want to draw their card from, and no victory points are given out. If there are less knights than there are cities, the player with the least strength has to choose a city to be pillaged and turned back into a settlement, and in the case of a tie all tied players must choose a city to revert back into a settlement.
So as you can now see, Cities and Knights has a lot to offer to those looking for more content and rules but wanting to keep the flavor of the original game.