Where to build markets - only in fertile lands?

[Dead Guy]

AFAIK, the fertility doesn't affect the amount of food the province produces, only the income from farming. The 4 levels of farming always give you 1-4 units of food. You'll just have to leave some castle towns as forts and only build an upgraded market there, no troop production. Specialize the regions a bit, like in civ.

To clarify, it's when the global food goes below 0 that people become unruly.