Legitimate or not, Joan remained the only child that could be remotely attributed to Henry IV of Castile and the king was very fond of the child. He even made the nobles of Castile swear alliance to her and promise that they would support her as queen. After a few unsettled arrangements, that included French and Burgundian princes, Joan was promised in marriage to her uncle, king Afonso V of Portugal, that swore to defend her (and his own) rights to the crown of Castile. But when Henry IV died in 1474, nobody took Joan's cause seriously and the crown went to Isabella I of Castile, her aunt. In the following year, Afonso V married Joan and prepared for the fighting. In 1476 he invaded Castile, but was defeated in the battle of Toro by Ferdinand II of Aragon, Isabella of Castile's husband. After this, Afonso V tried to procure, without success, an alliance with the king of France. In 1479, the king of Portugal gave up on the pretension and signed a treatise with the Catholic kings.
Following the death of Afonso V, Joan of Castile refuse to marry Juan, the son of Isabella and Ferdinand and had to retire into a convent in Coimbra. The princess lived in Portugal for the rest of her life, dedicating her life to charities and social works. Her denial to get herself involved in political intrigues and her devotion to God and the poor, gave her the nickname of excellent lady.