I have never seen the AI attack a fort. This is an issue because in my campaigns the AI continually tries to siege settlements with huge garrisons due to forts with armies that can't take the settlement. The besieged settlement then just attacks the besieging army and defeats it or forces it to withdraw. The next turn the withdrawing army will attempt to besiege the same settlement and be forced to withdraw again. This makes the AI functionally incapable of attacking settlements in the late game unless they are one of the few factions that can muster 2 or 3 full stacks. Please make AI armies prioritize destroying forts prior to attacking settlements to give it better odds of conquering territory.