Where is the ghost first spotted in hamlet?

In Shakespeare's "Hamlet," the ghost of King Hamlet is first spotted on the battlements of Elsinore Castle by the sentinels Francisco and Barnardo, along with Horatio. They see the ghost during a night watch and decide to inform Prince Hamlet about the apparition, believing it bears a message about the late king's death. This encounter sets off the chain of events that drives the play's plot.