I think the best answer to castle spaming is to make a default fortress really worse than everything else, as well as slow to build (I suggest as the default castle to add the palace... a fortress with a cost of 400, needing 5 turns to be built -lots of artwork needed-, defence 25, food 100, admin 10 but takes 30 on surrounding provinces -courtiers living in a palace are corrupted persons-). The watchtower should have a cost of 40 or even 60 as it's the easiest to build and cheapest fortress, so castle spaming may be still a strategy, but with an initial cost.
Another good option is to delete Caelum.
