I've been thinking about some of the issues raised as I've been thinking about seige rules.
I've really been thinking of seiges as campaigns.
In this context, I wonder whether the ideas contained within real time strategy games might not be useful. Consider that a tunnel or trench might need so much wood per inch on the table. That forest or town might yeild a certain quantity of wood, but this would have to be processed into useful material via a Mill which would itself cost so many "Wood" and so many Ducats/ Loius d'Or. A town would yield so much Grain, but a bakery would be needed to process it into Bread.
For our Army to survive at full fighting strength and morale it would have to consume so many "Bread" and "Gold". Gold might be bought at towns for "Gold". An area of land might yield so much "Forage" to feed the cavalry for so may strategic turns dependant on type as well.
Likewise, goods might be brought from distant locations by Barges or Carts, each of which could carry so much Bread or Wood or even Gold to pay the Army with.
The beseiged would have a stockpile of Wood, Bread and Gold and would draw upon then to supply the garrison and repair the defences.
You could attempt to spin out the Food supply by going to half rations with the concomitant reductions in morale and Build effectiveness and a rise in the rate of Desertion. A fortresses Horses might also be converted into "Bread" but a Butchers might need building.
Any thoughts on this model?