MADRID — The death of Don Carlos, Prince of Asturias, at the age of just twenty-three has reignited one of the darkest debates in European royal history: whether generations of unchecked inbreeding among the Spanish Habsburgs produced a heir so physically deformed and mentally unstable that his own father, King Philip II, was forced to imprison him in a fortified tower and effectively erase him from the line of succession.
Don Carlos was born on July 8, 1545, in Valladolid, the son of the future Philip II of Spain and his first wife, Maria Manuela of Portugal. His mother was seventeen years old. She died four days after giving birth following a massive hemorrhage.
The infant survived, but the genetic inheritance he carried would haunt every day of his short and troubled life.
His parents were double first cousins. Carlos’s maternal grandfather and paternal grandmother were siblings, as were his other grandmother and grandfather. Philip and Maria Manuela shared roughly twenty-five percent of their DNA, making them the biological equivalent of half siblings rather than husband and wife.
The prince had only six great-great-grandparents instead of the usual sixteen.
One of his living ancestors at the time of his birth was his great-grandmother, known to history as Joanna the Mad, sister of Katherine of Aragon. The Habsburg family tree had folded in on itself so many times that modern geneticists would later identify the dynasty’s distinctive facial deformity, the so-called Habsburg jaw, as a direct consequence of centuries of consanguineous marriage.
From the moment of his birth, physicians and courtiers noted that something was wrong with the royal infant. One leg was shorter than the other, a defect that would affect his posture for the rest of his life. Portraits of the prince were diplomatically adjusted to make his features appear more regular than they actually were, a common practice among court painters serving the crown.
A far more accurate picture comes from the Imperial Ambassador Adam von Dietrichstein, who described Carlos in 1564, when the prince was eighteen years old. The ambassador wrote that Carlos had brown curly hair, a long jaw and a heal on his face, one shoulder slightly higher than the other, a sunken chest, and a small lump on his back at waist height.
Dietrichstein further noted that the prince’s left leg was much longer than his right, that his legs were weak, that his voice was harsh and sharp, and that he had problems speaking, with words coming only with difficulty from his mouth. This was the boy destined to become the next King of Spain, ruler of the world’s first global empire.
There were illnesses too. The child suffered repeated fevers that turned his skin yellow, and he endured bouts of what physicians at the time believed was malaria. Despite his physical ailments, there was at first no question that Carlos was the heir.
He was recognized as the successor to the thrones of Castile and Aragon, and his inheritance rights were laid out in formal treaties.
His father’s marriage to Mary I of England in 1554, when Carlos was nine, produced a treaty that explicitly addressed the prince’s rights. Had Philip and Mary produced a child, that child would have inherited Burgundy and the Netherlands, not Carlos. Mary would be his stepmother for four years until her death in 1558, though the two never met.
There was talk of marriage for Carlos as well. Elizabeth Tudor, the future Elizabeth I of England, was floated as a potential bride, though given her well-known aversion to matrimony, it is difficult to imagine she would have considered for a second a Spanish Catholic fourteen years her junior, physically deformed, already rumored to be mentally unstable, and her stepnephew.
Mary, Queen of Scots, was another potential bride. But it was to Elizabeth of Valois that Carlos was actually engaged in 1559, only for his father to then marry the thirteen-year-old girl himself in February 1560, much to Carlos’s chagrin. Another mooted marriage was to his cousin Anna of Austria, but that never came about either.
Philip would go on to marry her too, despite the fact that she was his niece.
More serious than his physical ailments were Carlos’s mental health problems. From childhood it was clear that he was not stable. When his grandfather Charles I abdicated in 1556, he met Carlos on his way to the monastery of Yuste, where he planned to retire, and was deeply unimpressed with the eleven-year-old boy.
Aside from the physical problems, Charles found him badly mannered, disrespectful to his aunt Joanna, and poorly educated. He even suggested having the boy kept at Tordesillas, where Charles’s mother Joanna the Mad had languished for half a century until her death the year before. It was a chilling suggestion from a grandfather who knew exactly what mental instability looked like.
Other stories from Carlos’s youth tell of cruelty toward animals and humans alike. He had creatures roasted alive. He made a shoemaker cut up and eat a pair of shoes that were not to the prince’s liking.
His gluttony and limited intellectual capabilities were commented on widely. One Venetian observer wrote that he wished neither to study nor to take physical exercise but only to harm others.
He was proud and utterly unmanageable, and he showed himself unable to carry out royal functions. His behavior on February 22, 1560, when he received the oath of allegiance of the Cortes, the Grandes, and the cities, was an embarrassment to his father. The boy kept refusing to let some of the nobles and attendants kiss his hand until Philip forced him to do so.
The king had considered making the boy governor of the Low Countries, but it was by now evident that this would be a terrible idea. Carlos was completely unsuited to any kind of command, and the position was given elsewhere. Still, Philip hoped that his son would improve with time and a better education.
Carlos’s dynastic position was still secure, and he was sent to the University of Alcala de Henares with councilors of state to attend and instruct him.
It was while he was there that a serious incident occurred which only made his mental state worse. On April 19, 1562, while on his way to see a serving girl, Carlos fell down some stairs in his lodgings and suffered a serious injury when he hit his head against a closed door at the bottom. He was knocked unconscious, though some accounts say he initially remained awake.
Whatever the case, he was soon suffering from severe fevers and partial paralysis, and he slipped into a coma on May 5. Philip came to be by his son’s side, and the doctors did all the things doctors in those days did, including bleeding him, cupping him, applying balms to the wound, trying to clean it, and attempting to trepan the prince, which means boring a hole in the skull.
The firsthand account of his doctor states that although this procedure was begun, it was not completed. Upon opening his scalp, the doctor wrote, he found the skull white and solid, and there began to come from the paracity of the bones some small drops of blood very red, and he stopped trepanning. It was evident to the eye that the skull was not injured, nor the internal part corresponding to that place.
Lucky Carlos, one might say. Having his skull drilled into was the last thing he needed at that point. Another remedy, and the word is used very loosely, was to have the dead body of a local saint called Fray Diego brought in and laid up against the unconscious prince.
That man had been dead for a century by the way.
Let us all pause for a moment and consider how unhygienic it must have been to put what was left of him on the bed. Apparently he had barely decayed, and that was one of the things that made him so special. Many credited this move with an apparent miracle, for despite being almost dead, and in fact his father was told there was basically no hope, Carlos suddenly began to recover.
His pulse strengthened. He woke up. He was coherent.
He was back on his feet by mid-June. What his actual injury was remains unclear to this day. A concussion might produce some of these symptoms, but fever, coma, and paralysis suggest something more serious, perhaps mild brain swelling that was severe but not enough to 𝓀𝒾𝓁𝓁 him.
Medical historians continue to debate the question.
Fray Diego, by the way, was at Philip’s request canonized by Rome as a saint of Alcala, and it is after him that the American city of San Diego is named. The prince’s recovery was to the delight of his father and Spain, but this delight must have been tarnished with time as it became evident that his physical and mental health were as bad as ever.
The prince continued to take severe fevers that concerned his doctors. There were rumors that he was impotent, though he certainly made many advances on women, randomly grabbing them in the street and kissing them. Of course, the fall had happened on his way to see a serving girl.
Other issues were more severe. He mistreated his father’s favorite horse so badly, having forced the servant in whose care it was to let him ride it, that the poor animal died.
He showed an unhealthy devotion to his young stepmother and former fiancee, Elizabeth of Valois, to the point that later writers invented a story of an 𝒶𝒻𝒻𝒶𝒾𝓇 between them, though there is no real evidence of this. When he was walking on the street and a little water that someone threw out their window landed on his head, he completely overreacted and ordered his guards to burn the house to the ground and 𝓀𝒾𝓁𝓁 all the inhabitants.
Fortunately, they returned to him and said that the sacrament was being given to a dying man inside, and so they felt it was best not to 𝓀𝒾𝓁𝓁 everyone. Luckily for those involved, this explanation mollified this mercurial and dangerous young prince. By 1565 he was also increasingly desperate to leave Spain, either to go soldiering or to be seen as a ruler and diplomat.
He showed a special interest in heading to the Low Countries, possibly to stir up discontent against his father, with whom he had an increasingly fractured relationship. Philip was not having it. When the Duke of Alba was given a post in Brussels in April 1567, Carlos was so enraged that he had not been given the honor himself that he threatened and possibly even tried to 𝓀𝒾𝓁𝓁 the duke.
Sources differ on the details, but one story says he flew at him repeatedly with a knife and Alba had to hold him off. He expected Philip to go to the Low Countries and take him along, but when the king canceled the trip, his son became openly disrespectful to him and began to plot to leave the country without consent, which would have caused a political crisis and perhaps all-out war across the Spanish Empire.
He strongly insinuated to his confessor that he planned to 𝓀𝒾𝓁𝓁 his father. When Philip learned from his illegitimate half brother at Christmas 1567 that the prince was planning to escape to Italy, he began to contemplate drastic action. When Carlos’s departure seemed imminent, he took an incredible step in the middle of the night on January 18, 1568.
The king plus four members of state and additional guards crept into Carlos’s bedchamber. Philip was wearing armor and a helmet, which tells you a lot about what he feared his son might do. They quietly removed all the papers and weapons in the room, and when Carlos awoke he was reassured that he would not be killed but was placed under immediate house arrest, though he begged to be murdered rather than imprisoned.
All heavy objects in the room were taken out, probably for the protection of Carlos himself as well as those who were to have the care of him, and the windows were nailed shut. He was later moved to another even more secure room. He would now remain imprisoned in the Alcazar of Toledo for the rest of his life, in more or less solitary confinement save for his guards, who were to ensure he was never left completely alone, and the occasional visit from a doctor or barber.
His reading material was limited to religious texts, and he was allowed to hear Mass through a small hole made in the wall. It cannot be emphasized enough what an astonishing step this was on Philip’s part. He had no other sons at this point, just two infant daughters.
To have to admit even just to himself that his own heir was so unstable that he was a threat to his father and the realm, and could not be trusted to ever rule, would be a difficult move to make even for a modern parent.
In the sixteenth century it was almost unheard of, and a humiliation to Philip and the entire royal family, though equally most were surely glad not to have to deal with Carlos’s violent moods and vicious tendencies anymore. The queen, mind you, had always found him to be very kind to her, probably because he was infatuated with her, and she cried for two days when she heard the news, until her husband basically told her to pull herself together.
Philip tried to control the narrative and cover up the worst of his son’s behavior, telling the Pope that despite the rumors Carlos was neither heretic nor was he planning rebellion. He was simply, in the king’s words, completely lacking in the capacity needed for ruling over states. The king’s actions had therefore been for the good of his realm and in service to God.
To his sister Empress Maria, he admitted that his course of action had greatly grieved him. In other courts, where the seriousness of the prince’s behavior was not fully understood, it was believed by some that he was merely being punished to bring him into line. Philip, though, knew from the outset, he said, that his son could not be healed and that the move was permanent, and he said as much in a letter to his brother-in-law.
The king stayed in Madrid, apparently fearful of an uprising which would free Carlos, and held a tribunal to establish that his imprisonment of his son and the prince’s exclusion from the line of succession were just, though documents giving details of this body’s findings have now been lost to time. As it turned out, though, this confinement did not last all that long, and exclusion from the line of succession was not necessary.
Carlos, unsurprisingly given the conditions of his imprisonment and his general temperament, went even more demented now that he was confined. He quickly went on hunger strikes, then indulged in his old gluttony at another time. He swallowed one of his diamond rings, thinking that the stone would poison him to death.
He covered his bed in snow. His determination to ruin what little health he had quickly paid off.
In July 1568 he fell ill. Philip went to see him one last time to give his troublesome son his blessing. Then Carlos died on July 24, aged just twenty-three.
He was buried with full royal honors in the Church of San Domingo in Madrid, then moved to San Lorenzo in 1573, where most Spanish royals are buried. Philip was reportedly distraught at how this whole sorry saga had ended, and the court went into official mourning for a year.
Later fanciful accounts which sought to tarnish Philip’s reputation claimed that the prince was a secret Protestant and was murdered on his father’s orders, possibly by poison or even beheading. But while his death was certainly convenient in many ways, motive and opportunity do not equal a certain crime, and there is no evidence to support these stories.
Carlos was a very unwell child and young man who was a danger to himself and nearly every other living being who came into contact with him. He died professing the Catholic faith and in front of plenty of witnesses. Had Philip wanted to 𝓀𝒾𝓁𝓁 him, there would have been no point in locking him up first.
The deed could have been done quietly in January 1568 and put down to the prince’s known bad health.
Philip had no male heir after Carlos’s death, and his wife Elizabeth of Valois died later that same year at the age of twenty-two after a miscarriage of another girl. Philip, as mentioned, later married his own niece Anna of Austria, having clearly still not learned the dangers of incest, and only one of their children survived infancy, a boy who went on to become Philip III in 1598.
The degree of inbreeding in the family was at least partly to blame for Carlos’s ailments, both mental and physical. But the illnesses he contracted during his lifetime and the near-fatal head injury of 1562 may well have played a part in his dangerous behavior as well. Modern medical historians increasingly favor a combination of factors: severe genetic loading from generations of royal intermarriage, possible congenital neurological damage, and traumatic brain injury.
The case of Don Carlos remains one of history’s most stark warnings about the consequences of dynastic inbreeding. The Spanish Habsburg line would eventually die out entirely with Charles II in 1700, a king so disabled by generations of consanguineous marriage that he could not produce an heir, bringing an end to one of Europe’s most powerful dynasties. The tragedy of Don Carlos was merely the first chapter in that long, slow genetic collapse.