Macbeth's battle strategy in act 5 scene 4 is suggested by Old Siward :
We learn no other but the confident tyrant
Keeps still in Dunsinane, and will endure
Our setting down before 't.
Even after the junction of the two armies, the defection of thanes, and the desertion of common soldiers, Macbeth remains confidently confined in the castle of Dunsinane, waiting for the the rebel command, led by Malcolm, to launch its offensive. Since most of the nobles and soldiers of Scotland have deserted Macbeth, and the remnants of Macbeth's garrison have already gone over to Malcolm in their hearts, the tyrant can not come out in the open country to initiate the attack. Furthermore, Macbeth still believes in the prophecy of the third apparition that the Birnam Wood can never come to Dunsinane, and so he shall remain invincible.