The mythology of Tyndareos and Leda is quite interesting. Zeus seduced Leda in the guise of a swan. After a suitable period of time, she gave birth to two mortal children: Clytemensta and Castor; and laid two eggs which hatched as Helen and Polydueces (Pollux). The eggs hatched first. Thus, Helen is the elder daughter. Castor and Polydeuces are killed. This makes Helen the heiress of Tyndareos. Although she cannot rule Sparta, her husband can through her inheritance as a quasi-regency for Helen's son who inherit as a direct descendent of Tyndareous. Hence, whoever is Helen's husband rules Sparta and sires her son. She is important to the Trojan War cycle not as a major player but as a conduit of inheritance.