The audience knows from the previous scene that Lady Macbeth is
plotting Duncan's death, so the castle is far from a safe and
peaceful place: it is a deathtrap.
The audience may feel a sense of dramatic irony as they watch Duncan enter the castle, knowing that he is walking into a trap set by Macbeth. This creates tension and suspense as the audience is aware of the impending betrayal that Duncan is unaware of.