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.
