I have been wanting to play C&K but did not have time to read the rule book. It is daunting unless you have a real physical board to learn on. So I went out an bought the Expansion and read the rules and played it a few times and the light came on. This is actually very similar to the normal game, with extensions that allow you to improve, destroy, defend, and ultimately convert your cities into metropolises. Knight cards have changed into actual knight pieces that are purchased the same as the basic development card resources ore and sheep + activated with wheat. We still have 6 victory points, it is just that they are earned by the knights in battle. Very ingenious. All the changes seem to be logical extensions of the basic game given that you want to do more with cities.
So my one complaint is that my friends don't have time to read a book before playing their first few games. So I put together a two page concise set of rules for my friends and it seems to be working well. If anyone is interested, here is the link for it. https://www.dropbox.com/s/hjxuw3lluxwv30k/Catan Cities %26 Knights Rules.docx?dl=0