when should you build a new fortress?

[AreaOfEffect]

Again, it highly depends on the nation and the need.

If it is to hold a choke point or defend an important magic site, then some nations are better off putting that 800 to 1200 gold into PD. Other nations, like late-age Man, have horrible PD but are excellent at defending castles.

[Xietor]

When NOT to build a fort is just as important.

Never build a fort that you cannot defend. And never build a fort and leave it lightly defended. You can get away with lightly defending forts if you are an astral nation and have gateway researched, but it is a good practice to keep all forts well guarded. Nothing is worse than spending gold to build forts for your enemy.

And typically I hold off on building forts until i scout my neighbors and see if they are building forts I may want to take.