there is an allusion to "Hamlet's Father". what is the connection?

Check this quote out,

Dickens is re-iterating that ‘Marley was dead, to begin with’ (I love that comma in the opening sentence and the way it emphatically opens the mystery and the magic now unfolding). That Marley was dead ‘must be distinctly understood, or nothing wonderful can come of the story I am going to relate. If we were not perfectly convinced that Hamlet’s father died before the play began, there would be nothing more remarkable in his taking a stroll at night, in an easterly wind, upon his own ramparts, than there would be in any other middle-aged gentleman rashly turning out after dark in a breezy spot – say St Paul’s Churchyard, for instance – literally to astonish his son’s weak mind.’ And so we are plunged into a world of vengeful phantoms, spirits, or goblins damned, where Jacob Marley’s ghost is sure to walk abroad.