So I'm finally playing through a real game with C3X and the AI is besieging my outlying cities with catapults and AHHH THIS CHANGES EVERYTHING
Yup...It's a bit different. But, on the other hand when you take out such a stack, you get their artillery type units.
I just finished a playthrough on the "Napoleonic Europe" Conquest at Emperor, using C3X R9. The pRNGods had deemed that this time I should lead France, so it was a relatively easy win: didn't even need to cash-rush much, despite the hideous shield-cost of the Imperial Guard and -Cavalry units.
And by the end of it, every one of my ~60 Grand Batteries was foreign, including several which had clearly changed hands more than once: among my collection were also Danish, Prussian, and Spanish GBs, despite my having been allied with all 3 of those nations pretty much from beginning to end.
(Prussia survived only on sufferance; although I mustered more than enough firepower to crush her, the Ottomans and Russians were simply more insistent that they should have my attention — and lose their towns)
Getting stuck as e.g. Spain or Austria-Hungary would have made the game a lot more difficult, though...