Population decreases when you recruits.

[Jeanne d'arc]

Well your not wrong, every unit u recruit drains population.
I always tend to recruit peasants in recently enslaved towns and transfer them to towns that need population boosts to achieve the next level, especially towns where u focus on military cause u want them to produce the best unit available for your faction to have as soon as possible.The towns u drain will make good money for a litle longer this way.

[Vikings]

Ah thank, just wondering if that what i though when i was playing. Kinda hard to keep track what is going on in the cities. I guess the best way is have maximum orders without going into revolts and set the growth rate to 0.0% another word try to increase taxes.

Any see how things work. And yes sometimes, esp. at the beginning you want your little ol' cities to grow, but on the other hand need more troops for the missions as well defending.

-Vikings

[Red Harvest]

And earlier, others made note that you could recruit peasant armies from large cities to move to small ones then disband them. When you disband the units add to the city population. I sometimes disband a few units just to get to the next city level early.

You can't build units that will drop population below 400.

[Alexandr III. Biges]

With this method you may actually "buy" population from your enemies, I mean bribing their armies, of course, if they are composed of units you may use. In case of rebels they are usually usable and especially peasants are cheap to bribe :)

[*Ringo*]

It's worth remembering that Mercenaries DON'T get added to your population when disbanded, this prevents the player from recruiting all mercenaries to boost population growth in sparse areas. I have personally tried bribing armies to disband, though i've stopped bribing all together for the moment.

*Ringo*