What if melee units could pick up a Battering Ram promotion in cities with siege workshops like ships get the Skeleton Crew promotion? It would give +5% bombard, -40% strength, and a Drop Ram spell that would get rid of the promotion. That way if you don't have time to build a catapult, you can just have some melee units grab Battering Ram promotions. On the downside, you would be severely weakening three units for every catapult you replace.