At the heart of romance lies a game of attraction - which some people play better than others. And the most successful at getting their heart's desire, are those people gifted in the art of seduction.
A siren learns how to take control by embodying the male fantasy. In male dominated society, this is one of the few ways a woman had of exerting influence. In Cleopatra's case, although she was a queen, to gain his protection she needed to manipulate one of the most powerful men in the world, Caesar. Not only that but Caesar had had numerous mistresses in the past, whom he quickly grew bored of, to return to what really motivated him - politics and warfare.
She seduced the Roman leader by speaking of reviving the glory of Alexander the Great and how together, they could rule the world like gods. She dressed as the goddess Isis, and surrounded Caesar by decadent opulence and yet when he felt secure with her attention she would turn cold or angry, such that just as if fighting a battle, he had to find a strategm to win her over.
So what makes a siren? Not necessarily physical beauty. Again, Cleopatra's allure seems to have been in the theatre she created around herself, how her make up and costume changed each day to keep people entranced. She kept her admirer's guessing, never let them feel on safe ground but always working to please her. What she realised was that by using sheer physical presence, her admirers could not grow bored and neither would they see who she really was.