Economy
Goods Economy:How about something simple for an economy. Villages produce certain goods based on their location and faction (coatal cities produce salt, etc.). A villages prosperity affects the quantity goods it produces. If there was a population system, the population of a settlement could decide how much goods are consumed. If you plan on making a disease system, you could also incorporate a famine system, which affects how many goods are produced and consumed, the population system that I specualted on, and the village's prosperity. Castles do not produce any goods, but the amount of goods consumed depends on the number of troops stationed there, and a "possible" population system. Towns, produce more manufactured goods (velvet, etc.) but a still a bit of raw goods, like food stuffs. All speculation about famine and population system applies to towns. The goods trader's wares are based on what goods are produced in the surrounding village, as well as the town. The amount of goods for sale is based on the prosperity of the town and surrounding villages.
Weapons and Armour Economy:This is somewhat based on the buildings in the town and Castle(s) in the region. Some buildings that would impact weapons production:
-Weaponsmith: allows <insert weapons> to be sold in the <town>
-Armourer: allows <insert armour> to be sold in <town>
-Leather Tanner: allows leather armour to be sold in <town>
These are just some examples off the top of my head. The amount of said merchandise produced by said building depends on the castle's/towns prosperity (which is impacted by the Goods Economy).
Horse Economy:This is impacted by building. Each village and town with a stable built in it can provide the horse merchant with basic horses. If there is a stable and a leather tanner, mid-level horses are sold. If there is a stable and an armourer in the region, then highly level horses are sold. The amount of horses sold depends on the prosperity of the region.
This is a system that is simple, yet interconnected, and, in my opinion, workable. Though I don't know how much work t would be. What do you think?