[quantum_mechani]

You can't really have too many forts, just so long as you have enough money left over to keep up in research. Don't obsess too much about where to place them, late game often ends with more places forted than not.

Province defense 1-4 is a must to avoid scouts and Call of the Winds stealing your province, past that only buy more if you have good reason to expect an attack that PD can repel or help repel.