* You need at least 500 denarii to construct one
* Only generals (IE family members) can construct forts or watchtowers
* You cannot build a fort on certain terrain types, EG river crossings (fords), bridges etc
* If you abandon the fort but wish to keep it, you must leave at least one unit as a garrison to avoid the fort from being pulled down
* A fort can hold out for a maximum of 3 turns during a siege
* A fort defences are pretty minimal - all you get is usually a defensive tower at each corner of the fort (so 4 in total)
* Use forts to stranglehold mountain passes, bottlenecks and other strategic positions on the campaign map

That's about it