How is gameplay improved in the Cities & Knights Expansion?
I often describe Cities and Knights as "the version of Catan where every limitation has a way around it." You can only hold seven cards… unless you build these walls. You only get cards when your numbers roll… unless you have an aqueduct. Etc.

Cities and Knights takes Catan and adds a layer of interesting shortcuts, passthroughs, special cases and escape hatches on top. There's slightly more to learn, but it's still a simple game. But the increase in complexity is outweighed by the vast increase in interesting mechanics between players and events in the gameworld.