If you are unfamiliar with the Catan series please read the review, 'The Settlers of Catan' first. This will give you a basic overview of the game and save us time here.
Welcome back. So what is different? Well each player starts with 1 city and 1 settlement. The city also comes with a flip chart and this displays 3 sections, which represent the commerce, political and military development of your city. Each of these sections must be developed during the course of the game and doing so will gain you great advantages. But they naturally come at a cost. Cities & Knights (C&K) introduces 3 new commodities in cloth, coin and paper. These can be earned through the terrains pasture, mountain and forest and the cities now have a changed production. They still earn 2 bricks and 2 grain (in hills and fields) but on the other 3 terrains they earn 1 resource and 1 commodity of the appropriate type. Commodities are handled using cards just like resources and this can cause added trouble when a 7 is rolled as they add to the size of your hand - beware the robber! The commodities also serve to spice up the trade options open to players.
The island in (C&K) is created in the same fashion but it is now surrounded by a border template and the top of the map has a new template added. This displays a track for the barbarian horde who will attack the Isle of Catan by ship. An additional dice is added to the resource die when they are rolled at the start of a player's turn. This dice depicts 3 castles and 3 black ships. If a black ship appears then the barbarian horde moves one step closer to the island. If a roll sees them make landfall then the player's must try to defend the island using their knights (yet another way to spend your valuable resources). I will keep the resolution of this phase a secret but if they are not defeated and you offer the least help, the barbarians will raze one of your cities, reducing it to a settlement. You can choose to lose a city wall instead if you purchased one (cost of 2 bricks) but these can be hard to acquire in some games. Therefore brick monopolies can be highly valuable.
Now back to the extra dice. The coloured castles relate to the 3 areas of your city. Every time a city upgrade is purchased, the appropriate section of the city card is flipped. Among other benefits, the new upgrade displays a set of numbers in a red dice. If your resource role shows the appropriate coloured castle and a matching number on the red dice (resource dice now consist of a bone + red dice) then a new progress card of that colour can be drawn. The old progress deck of Settlers is replaced by 3 new ones. Each deck relates to a section of your city. These cards create far more variability in the play. A total of 13 VP are needed for victory.

This version is much more involved than the original - some people will dislike this, whilst others will love it. The mechanics themselves are very neat and make for more paths to victory than the original. If you are a Catan fan then you will have trouble resisting this variant. It does take longer to play but for me the journey is strewn with greater challenges making the added time worthwhile. Bravo Mr. Teuber!