spamming castle all over feasible?

[Vynd]

I believe you are right that having forts of some sort in every province will maximize your output of gold and resources. As tinkthank says, though, the added resources and gold won't always be worth the cost of building the fort. It is also worth keeping in mind that you only need so much resources. Increasing the resources in some out of the way province where you won't build anything anyway does nothing for you.

So in practice, when you do see someone building forts everywhere, it usually has more to do with making their territory easier to defend than it does boosting output. Although the extra gold is certainly nice. Also, this strategy usually involves one of the cheaper-to-build forts like a Mausoleum or Watch Tower, not a Castle or something like it. The time and gold you have to invest in a Castle means it just isn't cost-effective to build them all over.