You are of course correct, I dont NEED to kill him, it would just be oh so satisfying. He is actually located in a chokepoint of sorts but it would be easy for me to just take the province and never try to assault the castle. But i am trying to be the dark lord of middle earth and all..and it just doesnt seem Dark Lordlike to tiptoe around a province instead of crushing it mercilessly beneath my boot. Plus the province is Angmar...and etimmu can't very well be my witch king of Angmar if i dont even control Angmar.
Its not a question of winning the game, that would be pretty easy from the position im in, its just a question of killing this damn sphinx in battle. 30 protection, 24 Mr, 9 astral, and 1200 hp is hard to take down.