Population is the people behind your empire. They live in your cities, eat food, and are the pool that armies are drawn from.
Growth and decline
Population is governed by food and housing, not built directly. A city with a healthy food surplus and housing to spare grows; one that runs out of food loses residents until supply recovers.
Housing caps growth
A city can only hold so many people. The ceiling comes from the city itself plus any housing you build in its districts, so growing your population means building housing as well as farms.
Spending population
Population is also a cost. Training a unit permanently takes residents to crew it, and a city can never spend its last resident. An army is always a real trade against the workforce that runs your economy.
Why it matters
Population sits at the center of the economy and your military at once. Every person is either feeding the empire or fighting for it, never both, which makes growing and spending population one of the constant balancing acts of the game.
This article reflects the game in active development and will change as systems evolve.