Thanks DGB246!

And people translate into... tax? And production and research? Assuming no unrest, if my city goes from 62 to 63 citizens - what benefits happen to the city?

...somehow I feel Faction Prestige should have some other side effects.

HenriHaki, 100 people = 10 research (less unrest), 10 production (less unrest), and (tax rate * 100) gildar.

So 100 people with 0% tax rate = 10 research, 10 production, 0 gildar.  Set tax rate to 10% (produces 80% unrest), and you'd get 2 research, 2 production and 10 gildar.

At certain thresholds, cities level up from population.  At 60 people, level 2.  At 200, level 3.  This usually provides a minor bonus like -10% unrest or so through a new building.

Lastly, population increases the area of control of the city.  I'm not 100% on this mechanic, though.