Sometimes the AI can ignore your forts.
sometimes I want to yell in face of everyone saying this "NO"
I agree with themeinteam rules are too stupid causing many confusion and problems that shouldnt be but stop saying "AI DOING SOMETING I CANT"
There are no rules special to AI
Problem is AI is programmed to see every path. AI doesnt play the game on screen you are playing . They play in a perfect perspective that very easy for them to understand pathing. So when they see a path that is available only for 1 moment that is you are not able to see why they can instantly take it if its worth it. After that moment unless they change their destination even if that path is now blocked you cant stop that because to save CPU movement when army is given command to move.
It can be stupid, it can be brutally hard to realize whats going on sometimes but its not "AI is cheating"
edit:About "cpu saving" part... you can do that too. If you ordered your army to walk throught a land that fort being built even after fort is finished your army will be able to pass it AS LONG AS you never click them to walk again
In short:AI cant ignore forts. Instead AI follows rules so perfectly it can see paths that players are unable to understand