wouldnt it be simple to allow construction of "Storage" houses. Which Litterly Stores houses. naturally they'd be placed all over, and you could select what type of material it would store (or all) that way in a siege it could be a great target for the enemy (who'd loot it/destroy it) so you'd naturally want your Stone supply to be within your walls, and perhaps a moderate mough of coin and food to buy additional soldiers, while your main food storage is located on the egde of the map near (human) farms, which you naturally has placed a couple of battalions to protect. it would also give incitement for people to try to protect their countryside, and not just abandon it if they can possible hold off the enemy.
"Orks is never beaten in battle, if we wins we wins, if we die we die so it don't count as beat, an' if we runs for it we don't die neither, so we can always come back for another go!"