The county of Borontuum had an extremely pitch-rich swamp which they built a castle on. Other counties got jealous and united to attack the citadel. There are 2 ways into the citadel, one is the large causeway with many blocks, the other is a small back door which is heavily guarded by archers. Since the back door is a direct route, when attacking the AI will always pick that way. I think attacking is more fun because you can pick either way.