The witches tell Macbeth that he will be defeated only if Birnam Wood comes to Dunsinane. This should give Macbeth a very good reason to *avoid* Dunsinsane, so that his enemies never focus on it. If I were Macbeth, I would probably burn down Dunsinane, and then move to another part of the country. Why does he, instead, fortify it? The prophecy does not say that he will be defeated when Dunsinane is overrun; it says that he will be defeated when Birnam Wood comes to Dunsinane.