A plain before the castle — the army arrives

A plain in front of the castle. The army has arrived at the walls of Dunsinane carrying the boughs they cut at Birnam. Malcolm gives the order to drop them. "Now, near enough; your leavy screens throw down, and show like those you are." The boughs fall. The size of the army is suddenly visible to the defenders on the walls; what looked, at distance, like a moving wood is in fact ten thousand English soldiers and the gathered Scottish lords with their forces.

Siward takes command of the battle order. He will lead the first attack himself, with his son. Malcolm and Macduff will follow with the rest. He delegates everything else to "the right-noble Siward and his son." Macduff calls for the trumpets — "make all our trumpets speak; give them all breath, those clamorous harbingers of blood and death." The trumpets blow. The army moves on the castle. The garrison inside, the audience already knows from the lords' march two scenes ago, has been holding only out of fear and is at the point of breaking.

The scene is roughly twelve lines long. It does what the previous scene set up — converts the witches' prophecy from rhetorical doom into an actual military formation. The walls of Dunsinane have a moving forest of soldiers in front of them. The army is no longer a metaphor and no longer an apparition; it is a unit drawn up for assault, with command structures, with trumpet signals, with a battle order. The next time the audience sees Macbeth, in the next scene, he is on the open field with his retreat cut off, fighting the army the audience has just watched form up. The trap Hecate prepared in Act 3 is now sprung; everything from here is the closing.