The Tragedy of Hamlet, Prince of Denmark, is a tragedy by William Shakespeare. Set in the Kingdom of Denmark, the play dramatises the revenge Prince Hamlet is instructed to exact on his uncle Claudius. Claudius had murdered his own brother, Hamlet's father King Hamlet, and subsequently seized the throne, marrying his deceased brother's widow, Hamlet's mother Gertrude. Hamlet is Shakespeare's longest play and among the most powerful and influential tragedies in English literature, with a story capable of "seemingly endless retelling and adaptation by others." The play seems to have been one of Shakespeare's most popular works during his lifetime and still ranks among his most-performed. It has inspired writers from Goethe and Dickens to Joyce and Murdoch, and has been described as "the world's most filmed story after Cinderella". The protagonist of Hamlet is Prince Hamlet of Denmark, son of the recently deceased King Hamlet, and nephew of King Claudius, his father's brother and successor. Claudius hastily married King Hamlet's widow, Gertrude, Hamlet's mother, and took the throne for himself. Denmark has a long-standing feud with neighboring Norway, which culminated when King Hamlet slew King Fortinbras of Norway in a climactic battle years ago. Although Denmark defeated Norway, and the Norwegian throne fell to King Fortinbras's infirm brother, Denmark fears that an invasion led by the dead Norwegian king's son, Prince Fortinbras, is imminent. The play opens on a cold night on the ramparts of Elsinore, the Danish royal castle. The sentries Bernardo and Marcellus and Hamlet's friend Horatio encounter a ghost that looks like the late King Hamlet. They vow to tell Prince Hamlet what they have witnessed.