The big problem is that the rules have been greatly modified in comparison to the main game (unlike the Seafarers expansion) and suffers from the balance.
Firstly, it seemed to often that a player has the same lost its city the first barbarian attack without he ever had the opportunity to defend. The barbarians were just too fast.
That would in itself is not so bad, but who is behind all, is grossly disadvantaged later in the game: The great and fair development maps no longer exist, they have been replaced by development cards, which can be obtained by developed cities the dice roll. Who has further expanded its cities, this gets more frequent progress cards. The causes of Fühende soon has a confusing pile with tons of cards in front of him, while the latecomers away empty.
For more variety at the table, I recommend the Seafarers expansion. Cities & Knights is me personally too far away from the original game.