 They were married with a clear prenuptial agreement on sharing power, and under the joint motto "tanto monta, monta tanto".He became "jure uxoris" King of Castile when Isabella succeeded her deceased brother in 1474 to be crowned as Queen Isabella I of Castile.The two young monarchs were initially obliged to fight a civil war against Joan of Castile (also known as Juana la Beltraneja), the purported daughter of Henry IV, and were swiftly successful.