Alfonso was born on 15 November 1453 in Tordesillas and died on 5 July 1468 in Cardeñosa. He was the son of King Juan II of Castile and Queen Isabel of Portugal, and the younger brother of Isabel. He was also known as Alfonso el Innocente (“the Innocent”) or Alfonso of Trastámara.

During La Farsa de Ávila, when he was only eleven years old, he was proclaimed king under the name Alfonso XII. This was carried out by members of the nobility who opposed the rule of his half‑brother, King Enrique IV. Although he was proclaimed king, he never truly exercised power and became a puppet of the nobility.

Soon after, a civil war broke out between the supporters of Alfonso and those of Enrique. In 1467, the conflict culminated in the Second Battle of Olmedo, which lasted from 3 p.m. to 8 p.m. and ended without a decisive outcome. Castile remained divided into two camps.

The court of Alfonso was established in Arévalo, where he lived in luxury for three years until his death. Alfonso died at the age of fourteen in Cardeñosa. The official chronicles mention the plague as the cause of his death, but public opinion and recent research point to poisoning.  According to the chronicler Alfonso de Palencia, he was poisoned either by, or on the orders of Pacheco, the Marquis of Villena.

His remains rest in the Cartuja de Miraflores in Burgos, beside his parents.