AI seems to be a bit better at siege to kill castles but they still often park their whole army under enemy castle and get killed by boiling oil. It seems that AIs threat detection regarding castles is flawed
This has always been an issue, ever since launch. I can exploit the AI for days since I know they will happily send everything under the keep and get burnt to death by boiling oil. Even AI villagers looking for stone or gold outcrops will do a conga line right under keeps and outposts.