Starvation of civilian populations is a time honored military strategy. Bonus if you starve them out, then use catapults to launch infected/rotten livestock over the walls.
"Chivalry was abandoned, too, if the fight dragged on, such as when Henry V had dead animals thrown into the wells of Rouen in France during his siege of 1418-19 CE. Catapults might launch manure and corpses in the hope of spreading disease amongst the enemy."
Which Monty Python reversed for comedy in Holy Grail: