From the first turn of a Civilization game the humble Settler is available to produce. With the benefits another city provides, what counterbalancing mechanics keeps a player from doing nothing but build city after city? In Civilization 4 the escalating maintenance costs could tank your economy. In Civilization 5 increasing technology and social policy costs could make the city a net negative for your empire. And in Civilization 6 there is no reason to stop expanding!
In Spirit of the Age I have a radical mechanic to discourage infinite city sprawl — what if you couldn’t build Settlers?