Final answer:
Horatio joins Marcellus on the castle walls to verify the sighting of a ghost reported by the sentries, a critical event that drives the plot of Shakespeare's Hamlet.
Step-by-step explanation:
The question 'Why has Horatio, who is not a sentry, come to the castle walls with Marcellus?' pertains to the characters in William Shakespeare's play, Hamlet. Horatio is not a sentry, yet he arrives with Marcellus, who is performing his duty to patrol. The play does not provide explicit details of their movements; thus, we must infer their intentions and actions through their dialogue. The context suggests that Horatio, along with Marcellus and other soldiers like Bernardo, share the objective of guarding the castle of Elsinore. It's implied that Horatio is there to substantiate or debunk the sighting of the ghost that the sentries have reported. In the broader scheme of the play, Horatio's role is crucial in validating the existence of the ghost, which is key to unfolding the central plot of the play. Horatio, who is not a sentry, comes to the castle walls with Marcellus in Shakespeare's "Hamlet" because they have heard about the appearance of the ghost of King Hamlet. They join the sentries on the night watch to witness the ghost for themselves. Horatio is known for his skepticism and rational thinking, so his presence at the castle walls signifies an intellectual curiosity to see if the ghost is real or if it might reveal something significant. This event becomes a crucial moment in the play, as it sets the stage for the unfolding tragedy and Hamlet's quest for justice.