Building Forts and Lake of AI intrest

I have built several forts at choke points and at the center of several key citys to act as a fire brigade. in several campaigns i have only had one fort under seige and then the enemy (Gual) ran away before any battles happened. Does the AI not understand the importance of attacking the forts. it seems to just by pass them only to be attacked later by the combined forces when it beseiges a city near by? This has happened repeatedly. Is there any way to upgrade the AI to take forts a little more seriously?