Macbeth plans to deal with the invading troops by forcing them to lay siege to his castle rather than meeting them outside. At the beginning of Act 5, Scene 5, he makes the following speech: "Hang out our banners on the outward walls. / The cry is still 'They come!' Our castle's strength / Will laugh a siege to scorn. Here let them lie / Till famine and the ague eat them up. / Were they not forced with those that should be ours, / We might have met them dareful, beard to beard, / And beat them backward home."
But shortly thereafter when a messenger reports the Birnam Wood seems to be approaching, he changes his mind about remaining in the castle. He cries, "Arm, arm, and out!--If this which he avouches does appear, / There is no flying hence nor tarrying here." In other words, he felt secure until Birnam Wood did come to Dunsinane, an impossibility; but once this happened he lost all hope and preferred to die in hand-to-hand fighting outside his walls.