Catan: Cities & Knights (previously The Cities & Knights of Catan) was released in 1998 as the third expansion for The Settlers of Catan. It shifts the focus of the original game away from expansion and toward city-building. Cities now produce commodities, can gain special privileges, and eventually become metropolises. The expansion is often considered the most advanced scenario for Settlers. Cities & Knights is notorious for its inability to combine well with other scenarios and variants, although there have been successful attempts. It can be expanded with the 5-6 player extension.
From the 3rd Edition packaging:
Hark! You must have The Settlers of Catan board game in order to use this game expansion.
Dark clouds gather over the once peaceful landscape. Wild barbarians, lured by Catan's wealth and power, maneuver to attack. Their massive warships loom against the orange horizon.
You must be strong! Barbarians attack the weakest targets, and the victim of their onslaught will be the player who contributes the least to the defense of Catan. Don't take any chances! Field your knights!
In The Cities & Knights of Catan, you engage in the defense of Catan and compete to build the three great metropolises of Catan. Each of these magnificent centers are even more valuable than cities and are safe from the barbarians. You must invest in city improvements, which you acquire using commodities of trade: Coin, Paper, and Cloth. If you improve your culture, muster your knights, and enrich your cities, you will be the master of the great realm of Catan!