Forts keeping away rebels is good.

My main purpose for forts though, is much like Professor Spatula.

It gives a place for units to be protected on moves;

and it serves as a great place for assembling and equipping an Army to send out to take another territory.

One fort, roughly equidistant between 3-5 settlements, gives an ideal place for infantry to come from one location, archers from another, and cavalry from yet another, and they can be set to automatically go there.

Also, if one of the cities has a foundry, I can look in the fort and find the ones who can get an armor upgrade.

In this manner, I can usually build up a full stack in about 4-6 turns, depending on where the fort is and what cities it is near, and actually send them all with the best upgrades in armor and weapons.

This will take a little longer of course if one uses Praetorian Cavalry, Urban Cohorts, and heavy onagers, but I am sure you get the picture.

Strength and Honor,

Celt Centurion