Material, money and salvage
Sixteen goods with a stock, a price and an appetite; workshops run recipes; the mountain feeds the city early and the city feeds on itself late, and the two crossings are tuned to happen at about the same time.
The whole of it, in one loop: the mountain is dug into stock, stock is built into the city, the city burns or is neglected into ruins, ruins are salvaged back into stock, and ships bring what the ground has not got. Early on the arrow from the mountain is thick and the one from the ruins almost nothing: a founding-era town robs a ruin only if it is much the nearer, and gets a quarter of it back. By the industrial age the two are comparable, and by the contemporary age the city is mostly feeding on itself, which is exactly when it has run the mountain down. The recovery rate per era is in the rules reference.
Goods exist as quantities in warehouses that are actual buildings in actual places; burn the warehouse and the grain is gone. Grain, fish, timber, stone, clay, brick, charcoal, coal, iron, steel, cloth, tools, glass, concrete, ale and luxury each have a base value, a bulk that sets the cost of shipping them, an era they first exist in, and whether they spoil. Nine recipes turn them into each other in workshops, mills, forges and factories: a kiln takes clay and charcoal to brick, a blast furnace iron and coal to steel.
Money is one treasury. Income is taxes on private income and the market’s own trade; costs are wages, relief, the garrison, public works and whatever the town buys abroad. Each person eats 0.85 grain a year (a tunable); a town that has not laid enough in loses people, and the chronicle says Hunger.
In the code: crates/sim/src/economy.rs crates/sim/src/city.rs