Isabella was born in the town of Madrigal de las Altas Torres on April 22, 1451. She was the Daughter of John II, king of Castile and his second wife, Isabella of Portugal. Through her life Isabella did many great things for the Catholic faith. Unlike most monarchs, she was not handed her power, Isabella had to work and fight to obtain what was rightfully hers.
The Castilian nobles had been constantly increasing their power through out their rule and had taken advantage of weak kings like Henry II and John II. Defeated at Olmedo the Castilians wanted to obtain the crown for the baby Isabella, not taking any notice of the king's daughter, Joan who was older. Throughout the rule of the Castilians they had cheated and lied and eventually stripped the crown of all its value. Isabella at a young age showed how great she was, when she did not accept the crown offered to her. When she denied the crown she also vowed that she would never accept the title of queen as long as her brother was alive. Soon though the king did something that no one could believe, he recognized Isabella as his immediate heiress to the throne, thus excluding Joan. .
In 1460 Henry offered Isabella's hand to Don Carlos, prince of Vienna and also to the king of Aragon, who wanted her to wed his younger son, Ferdinand. Many disputes and negotiations took place for Isabella's hand in marriage. The negotiations ended with the death of the prince of Vienna. Nine years later, in 1469, Isabella was married to Ferdinand in the palace of Juan de Vivero. .
In 1474, after the death of her half brother Henry IV, Isabella was made queen of Castile. Juana la Beltraneja, was supported by, and betrothed to Alfonso V of Portugal and also to Henry IV. Since she had been betrothed to Henry, la Beltraneja was the rightful heiress of his domains. The Archbishop of Toledo, who, while Isabella's father was alive had denied La Beltraneja as the rightful heiress, now defended her claims to the throne.