In selecting the subject of Hamlet, Shakespeare chose to work within this revenge play tradition. In fact, he was not even the first to write a play called Hamlet. All scholars are agreed that an earlier Elizabethan play on the subject once existed, conventionally referred to as the Ur-Hamlet, and almost certainly written by Thomas Kyd (1558-94)... The Spanish Tragedy has much in common with Shakespeare's Hamlet, including a ghost calling for revenge and a play-within-the-play, together with a sustained exploration of the themes of madness and suicide. --Paul A. Cantor, Hamlet, Cambridge, Cambridge Univ.Press, (1989), pp.27-28.

The tragedy of revenge has been classified as a definite, small subdivision of the Elizabethan tragedy of blood; and obviously, plays like The Spanish Tragedy, Antonio's Revenge, and Hamlet should be set apart.   --Fredson Thayer Bowers, Elizabethan Renvenge Tragedy, 1587-1642, Peter Smith, Gloucester, MA 1959, p.62.