Despite his worrying mental and physical state, as the only son of Philip II, Don Carlos was for a long time the sole hope of the dynasty. Although his death at the age of twenty-three released his father from an encumbrance in human terms, it left him without a male heir.
Born in 1545 as the first child of Philip and Maria of Portugal, who died giving birth to him, Don Carlos was a sickly child. He was physically deformed, with one shorter leg and shoulders of different height. In addition to these problems he also had a speech defect. His general poor health was worsened by regular bouts of fever which were probably due to a malarial infection. Following a fall when he was seventeen which resulted in a head injury he was subjected to trepanation: a hole was drilled into his skull to release fluid that was putting pressure on his brain. Don Carlos survived the operation but briefly lost his sight as a consequence.
In addition to his rather unprepossessing appearance, the prince was given to outbursts of violent temper and said to have been mentally feeble (although other sources attest to his good memory and wit). He was noted for aggressive behaviour towards those around him and enjoyed riding his horses to death. His waywardness was reinforced by his addiction to alcohol.
His high, boyish speaking voice gave rise to rumours that the young prince was impotent. In 1567 the twenty-two-year-old was duly subjected to a test under the supervision of his physicians that would prove his ability to father a child. The team of physicians, who treated the young man with all kinds of remedies, as well as the young girl who played the role of ‘guinea pig’, were richly rewarded after the experiment proved successful.
The physical and mental feebleness of his only son and heir was also the reason why the sons of Philip’s cousin Maximilian II, the later emperor Rudolf II and Archduke Ernst, were summoned to the Spanish court to be brought up there and groomed as potential successors. Since Maximilian’s wife, Maria of Spain, was a sister of Philip II, the two Habsburg princes were nephews of the Spanish king.
In 1567 relations between Don Carlos and his father reached rock bottom. The prince planned to flee to the Netherlands in order to ally himself with his father’s adversaries there. The plans were discovered and Don Carlos was interned on his father’s orders. Philip considered arraigning his son for high treason. The prince’s health deteriorated and he died in mysterious circumstances in 1568. He was said to have gorged himself on highly spiced pies and died of colic after drinking nearly ten litres of water to slake his unbearable thirst. Rumours that Carlos was murdered on his father’s orders were rife at the time.