Actually, a better way to solve city spam (rather than up army maintenance costs ... are u kiddin me holmes?) would be to have Total Worldwide population (of your faction) to cost food.

Like, 1 food per 200 global population, or something. Then, like another poster mentioned, have each Specialist Building cost a certain amount of "free population" ...

so you have used population and free population ... and only free population can be used to recruit soldiers (and many new buildings cost free population, turning them into used population) ... and all population, anywhere on the map, uses 1/200 food.

 

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if, somehow, a cities population dips through its "free pop" and somehow lowers the used pop ... then buildings that use "used pop" will lower in effectiveness.

Preferably at a % rate based on the entire Used population. (so if half of the used population die, then all buildings work at 50% effectiveness)

 

As far as City-Growth n Prestige go ... I feel that there should be not only Pop increase from Prestige, but also from Birth.

(Birth growth would start out VERY low or at 0, and then would slowly increase as the population increased, kind of like a shallow logarithmic curve)