Enrique IV's masculinity was faltering. After his dethronement in effigy at the Farce of Ávila in the summer of 1465, Juan Pacheco and his allies had called into question not only the king's right to rule, but also his Christianity, heterosexuality, and even his masculinity. As they fought against Enrique in a civil war, they championed the royal claim of Alfonso, the king's elevenyear- old half-brother, who was apparently sufficiently masculine. A few years into the civil war, though, the rebels lost their champion when the fourteenyear- old Alfonso died in 1468. With no viable male candidates remaining for the throne, the rebels rallied behind Enrique's half-sister, Isabel. By the end of the summer, the two sides had negotiated the Treaty of the Bulls of Guisando, whereby Enrique would remain on the throne but would accept Isabel as his heir. Enrique's daughter, Juana la Beltraneja, was cut out of the succession and Enrique ended his marriage with Juana's mother. These stipulations all confirmed Enrique's impotence, both as a ruler and as a man, both of which had been components in resistance to his rule in the first place.

The one other stipulation of the treaty was that Isabel would have to obtain Enrique's permission before she married anyone. She herself was placed in submission to the men around her, first by the noble supporters who hoped to control her and then by her half-brother, weak though he might be. Almost immediately, though, Isabel took her fate into her own hands by secretly agreeing to a marriage with her second cousin, the future Fernando II of Aragon. When the marriage was actually concluded in October of 1469, Enrique abandoned the terms of the treaty, insisting that his daughter would be his heir once again. When Enrique died in 1474, both Isabel and Juana were proclaimed queen of Castile. Two women would fight for the throne in the War of the Castilian Succession, a remarkably rare occurrence in medieval history. Isabel and her supporters would eventually prevail, but the reality of having a choice between two female claimants reflected a challenge to comfortable norms of gender in late medieval Castile.