This scene opens with the use of figurative language from Macbeth, who, "bear-like," knows he must "fight the course." This is juxtaposed with the enigmatic advice he has been given that only one "that was not born of woman" can kill him. Macbeth is heartening himself and telling himself that he can indeed continue on because such a person surely does not exist.
We know that Macbeth feels very sure of himself in this regard; Shakespeare uses the technique of the rhyming couplet to signify a sense of completion and conclusiveness in Macbeth's thought: "But swords I smile at, weapons laugh to scorn / Brandish'd by man that's of a woman born."
The stage directions are important in this short scene. The repeated use of entrances and exits and the continuous "alarums" underscore the chaotic nature of what is going on. The interchanges between characters are generally short parries, reminiscent of swordplay. At the end of the scene, we see a temporary lull which is, in fact, an example of dramatic irony: Siward says that the castle is "gently render'd" and that "little is to do." He does not know how fierce the fighting has been, nor that Macduff, who is indeed not "of woman born," will yet kill Macbeth in a pivotal scene.