Travel was difficult in medieval times, and it was customary to welcome strangers passing through. This explains Hamlet's wordplay in his response to Horatio's description of the encounter as "wondrous strange.” Horatio is remarking on the strangeness of this apparition, but Hamlet wants Horatio to welcome the ghost as he would any other stranger and not alert Claudius to its presence.

(Hamlet and His Father's Ghost, William Blake, c. 1806)