[Actuarian]

I agree with the previous posters in that forts are usually a more important economic decision than a military one. It can also be a good thing to build a fort in a place with good indie troops, like longbowmen.

If you do build castles defensively and you think an enemy might invade in a turn, make sure you choose if you want your troops to defend the province or hide in the castle. The default is to hide in the castle. I can't remember the defend province command (Is it patrol?), someone help me. Personally, I have a hard time remembering to choose and I invariably find province defenses destroyed and my fort beseiged.