Fire, plague and earthquakes
Each hazard rolls once a year with the shapes in the rules reference. Fire scales with thatch and a dry year, plague with crowding, earthquakes only shake the sites that shake, and all of them leave something in the ground.
Fire rolls against how much of the standing town still burns (everything of the renaissance and earlier counts as flammable) times the year’s dryness; a fire code cuts the exposure to about a third. It spreads through timber and stops at stone. Plague rolls against crowding and the age’s sanitation, which improves sharply from the industrial era; it lasts one to three years at a severity the era’s mortality scales. War needs a town worth marching on, a population over 150 and a threat above one, and then it is a siege: a blockade that takes the stores, an assault if the army outweighs the garrison, a sack that burns quarters and carries off the treasury.
Earthquakes happen only on the sites that shake, drawn once with the climate, and those shake often enough that everybody alive has seen one. Masonry is what suffers: a timber frame racks and stands, a stone wall cracks and comes down, and everything founded on wet ground does worse than the same building on rock. Mines cave in, wet ground throws sand up through itself, every banked hearth starts a fire, and the rubble stays in the street for the next century to lay its floors on.
The base chances are tunables; the shapes around them, the radii, the brittleness of old masonry, how much a sack takes, are in the rules reference.
In the code: crates/sim/src/events.rs · playground: events