The King, Cymbeline himself, is based on a British chieftain, Cunobelin, who reigned at the time of the Roman invasion.
Though once held in very high regard, Cymbeline has lost popularity over the past century. Some have held that, written late in Shakespeare's career, the play was a personal joke of Shakespeare's, parodying his earlier works. For instance, Iachimo (or "Little Iago") tries to fool the hero, Posthumous, into believing that his beloved Imogen has been unfaithful (as in Othello). The falsely besmirched Imogen fakes her death to weather the reverberations of this trick (as Hero does in Much Ado About Nothing). Some have taken the convoluted plot as evidence of the play's parodic origins.
Imogen is one of the very small number of great female roles in Shakespeare.