The knocking at the gate of Macbeth's castle is effective because it occurs just after the murder of Duncan when Macbeth and Lady Macbeth are both in a highly anxious state of mind. The two, having just killed their king, do not have the luxury of relaxing and calming themselves. Macduff is knocking at their gate because the king had asked him to arrive early that morning and waken him. Now Macbeth and Lady Macbeth must hurriedly wash their hands and, as Lady Macbeth commanded, get their nightclothes on so that it appears they've been awakened from their sleep. In reality, they haven't been to sleep yet because of the murder. Macbeth is so distraught, though, that he declares he cannot return to the king's bedchamber to put the daggers on the guards, which Macbeth forgot to do after he stabbed Duncan to death. Lady Macbeth chastises him, telling him that it's no big deal to have blood on his hands. She takes the daggers and puts them on the guards, thus getting blood on her hands. Both Macbeth and his wife have bloody hands when the knocking begins, so the timing of the knocking emphasizes the anxiety here as they have to clean up and try to appear as though they've just been awakened and have no idea a murder was committed in their house.