Sends supply carts into the field, so troops can eat, drink and build away from your roads.
Each cart is loaded with food, water, siege stones, ballista bolts, gears and building materials (logs, planks, metal blocks). Troops nearby eat and drink from it and take spare gear, allies’ troops too, and use its materials to build siege machines and ladders. A carter that gets tired or hungry parks the cart and goes home; the next carter picks it up.
Carts don’t need roads. Troops sent together with a cart march at the slowest one’s pace.