Here is the challenge from Civ games of old - can you win the game with just one city? You are given one settler in the beginning as per usual but you cannot build any more, nor can anyone else. No cities can be assimilated into another civlization by culture either.

The tech research has been accelerated somewhat so that you don't get bored. Also, on Chieftain level, the first six citizens are happy (instead of four), dropping one each difficulty level you go up until at Deity only the first citizen is content.