[Omnirizon]

I generally build forts to allow me to shuttle troops to frontlines quicker; allowing me to simultaneously better fortify a position and attack another.

I build first with respect to strategic placement and resource availability. Special troop availability may figure in as a secondary factor.

This said, early on I like to build a fort at least every four provinces. That may seem like a lot of fort building, but when you are able to consistently pummel your foe whilst they are trying to move troops to their frontlines from 6, 7, or even more provinces away, the benefit to you quickly becomes apparent.

I typically don't build a fort on a chokepoint, unless it happens to also be very resource rich and have lots of connections. However, I like to build right next to chokepoints, but I opt for whatever province can provide the best resource production and most movement options.

Forts are used to increase your production and mobilization speed, build them with that in mind.