Yeah so the reason for this OP was, a lot of the faction traits i've added have various monsters units and stuff to try and add a bit of faction diversity. Generally I cap these units though. In this case i've got "Ogre Alliance" faction trait.... it lets you train Young Ogres, Ogres and Bone Ogres... but you have to build Ogre Pits to get them. They are zero maintenance so you can only have so many. At the moment i've got this
<Description>An Ogre Pit allows Ogres to be trained in the city, however their presence causes some unrest among the populace.</Description>
<Provides>+3 Ogre Influence (+1 per City Level)</Provides>
<Calculate InternalName="Value" ValueOwner="OwnerCity">
<Expression><![CDATA[[CityLevel] + 3]]></Expression>
So the bigger the city the more "Ogre Influence" you have. Young Ogres require 2 Ogre Influence, Ogres 3 , Bone Ogres 4. That kind of thing.
I like using the size/level of the city as a factor, to try and encourage a bit of tall rather than wide empire building. I'd be temped to go for CityLevel * CityLevel = Ogre Influence or something.... so that you get more Ogre Influence (or whatever resource) on an exponential basis for city level rather than linear. I figure that would reward a tall build.