I am posting this here, though it is a general TW thing:
Many sources tell us that during the pre-gunpowder era, artillery was most times built during the siege and was not carried around with the army, being pretty uncommon in open field battles (except for the roman army).
Arty (catapults, onagers, trebuchets, etc.) could be built during the sieges taking several turns each piece, like ladders or towers. At the same time, arty could have a much greater impact on movement speed of armies, as well as higher upkeep and recruitment cost. This way, arty during open field battles would be less common, and using it would give a great advantage at a great cost on both manouverability and money. Sieges would also be tougher, as it would be much more difficult to break down the enemy walls.
Maybe some kind of high-tier logistics technology could give a boost on arty movement speed, allowing late armies to use it more effectively on the open.