Catan : Multiple extension combination

Question

I just bought the extension Cities and Knights (I don't know the rules very well at the moment).

I already have the base game and the Seafarers of Catan.

Is it possible to combine, on the same board, the Basic game of Catan, the Seafarers of Catan extension, and the Cities and Knights extension?

Answer 1

The rules for Cities and Knights (pdf) address combining it with Seafarers (page 13). They warn that only certain scenarios work well, and provide rules for things like how to treat ships as roads and how a knight can chase away the pirate. There is also a FAQ on combining the two expansions, which addresses a few points that the published rules don't cover.

One crucial point which they don't cover, in either the rules or FAQ, as discussed in an earlier question, is how many victory points to play to. It seems that the consensus is that when combining Seafarers and Cities and Knights that you should add 2 victory points to the victory conditions listed in the Seafarers scenario. See the other linked question for more detail.

Answer 2

Seafarers, Cities and Knights, and Traders and Barbarians are all expansions for Settlers of Catan. Every expansion requires the original game to setup and play.

The various expansions can be mixed. For example, Seafarers and Cities and Knights can be combined without trouble. In some cases the rules conflict - in which case adjustments need to be made. The Traders and Barbarians game manual describes some of the possible combinations. Many more combinations are listed on Catan.com:

Combinations with "Traders & Barbarians"
    The Fishermen of Catan - The Rivers of Catan
    The Fishermen of Catan - The Caravans
    The Fishermen of Catan - Barbarian Attack
    The Fishermen of Catan - Traders & Barbarians
    The Rivers of Catan - The Caravans
    The Rivers of Catan - Barbarian Attack
    The Rivers of Catan - Traders & Barbarians
    The Caravans - Barbarian Attack
    The Caravans - Traders & Barbarians
    Barbarian Attack - Traders & Barbarians

Combinations with "Seafarers"
    The Fishermen of Catan - Seafarers
    The Rivers of Catan - Seafarers
    The Caravans - Seafarers
    Barbarian Attack - Seafarers
    Traders & Barbarians - Seafarers

Combinations with "Cities & Knights"
    Catan for 2 - Cities & Knights
    The Fishermen of Catan - Cities & Knights
    The Rivers of Catan - Cities & Knights
    The Caravans - Cities & Knights
    Barbarian Attack - Cities & Knights
    Traders & Barbarians - Cities & Knights

You are not limited by these "official" suggestions. You could combine any set of Catan expansions, variants, extensions, or scenarios as long as you understand the rules well enough to resolve any conflicting rules before you start playing. That said, some combinations would be harder to resolve than others (like Cities and Knights + Barbarian Attack which have overlapping concepts with different implementations).