How to solve castling effect?

[Maltrease]

I don't think it doubles the resources. It only lets you use the full amount of resources available to the provence.

If I provence has 100 resources you would only be able to use 50 without putting a castle there.

However fortreses will draw their admin percentage from the full resource amount. So a fortified city next to the province would draw 50 resources... and another one on the other side would draw the remaining 50.

So if you place your castles correctly you would not be losing that many resources vs building a fortress in each provence.

This also allows you to draw the resources whre you need them (Heavy calvary for example) where if you built a castle in every provence you would not be able to do this.