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One way of limiting the extent of artillery in (sp) battles would be to give artillery a dramatically reduced per-turn campaign map movement than infantry (and of course less again than cavalry). So if you want to bring artillery/siege weapons to a field battle, you're going to have to crawl across Europe to do so. I know it's there to an extent in R:TW, but it could be more.
Even in the 1500's Cannon, trebs and all other kinds of heavy arty (i.e. stuff that you would use against a wall) were incredably heavy and difficult to move. I would be resonable imho to make heavy arty immovable so after it has been placed it can't be moved. The idea that you could use cannon to breach the first wall, storm it with your infantry and then move your cannon up to the breach and start blasting away at the second wall is silly.
In reality it would take days to disassemble it, dig a firing position, move the very heavy barrel, reassemble it and start firing again.