I'm at war wit OE, Novgorod is fully maintained fort. Why can they simply walk through it without having to siege it?
You say that the fort in Novgorod is active. It needs to be active before the AI army was given a movement order otherwise they will ignore it when they get there. This applies to human players too.
It also needs to be manned. If you activated it at the beginning of the month (in game) then there will be no garrison yet and so it will not exert a zone of control. This has the same effect as if it was not active when the movement order was given.
Assuming neither of those explanations apply here then what you are seeing is probably because of something people here refer to as the shortcut rule. It is believed that, to avoid crazy pathing, armies can move directly to any province that they could reach by a roundabout route as long as they could reach every province on the short route also via a roundabout route.
In your case, assuming the Ottomans have access to Novgorod, they could go via Pskov and Ingermanland to get to Noteborg and so the game allows them to go the short route. This rule is not an AI cheat. It applies equally to human players but it is not always obvious from the interface.
If the fort in Pskov were active then the roundabout route would not be available and, subject to whatever might be going on to the East, they would probably have been fort locked.
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