You could have the AI build a fort if there is a X amount of resources present or if those resources are not present within a 6 Province line then put it in one with most resources that is not adjacent to an enemy.

Also the behavior of an army to build a castle, are they going to take their main force and have them sit there while it's building or just place a small token force (with maybe some Provencials) and have the army more strategically placed.