Pity poor Robert Curthose – the firstborn doesn’t typically expect to end up the spare. He was an early specimen of what has become a stock character of royal drama, the failson. The historian Orderic Vitalis tells us how he ‘foolishly lavished on jugglers, parasites, and harlots’. Some of those harlots would steal his clothes in the middle of the night, causing him to miss the morning mass. This was no laughing matter: ‘the head being sick, the whole body faints, and the prince being weak, the entire country is in peril’. The ‘Venus of Sodom’ stalked Normandy when he was duke. The whole situation had to be put out of its misery – and so it was in 1106, at the Battle of Tinchebray, when Henry I captured his elder brother, thus restoring the personal union between England and Normandy that had existed under their father, William the Conqueror. Robert Curthose languished in prison for 28 years, till his death in 1134.

All this must have been quite satisfying for Henry: he had been imprisoned by Robert in Normandy many years before, and now the shoe was on the other foot. But putting a member of the royal family behind bars was not a straightforward business. Robert’s vengeful son, William Clito, was for a long time a thorn in his uncle’s side. Add to this that Robert had been a swashbuckling crusader, who played a part in the capture of Jerusalem in 1099, and therefore fell under papal protection. In 1119, Henry I had to reassure Pope Callixtus II that he was not keeping his brother ‘in fetters like a captured enemy, but have placed him as a noble pilgrim, worn out by many hardships, in a royal castle, and have kept him well supplied with abundance of food and other comforts and furnishings of all kinds’. William of Malmesbury was adamant that Henry’s conduct was just and proper. Indeed, he said, Robert ought to be grateful for his ‘brother’s praiseworthy sense of duty’, which ensured him the ‘continual attention of his guards, and plenty of amusement and good eating’. Robert’s captivity was, at least according to his captors, a cushy retirement.

Royals were seldom imprisoned prior to the Norman Conquest: the favoured method of dispatching rivals was execution or exile. Prior to becoming king in 1042, Edward the Confessor spent most of his life in Norman exile. Clearly that was for the best: his half-brother, Eadwig, had been executed by King Canute, and his full brother, Alfred, was brutally blinded and killed by Earl Godwin when he attempted to return. Godwin’s son, Harold – who himself briefly sat on the throne after the death of Edward the Confessor in 1066 – exiled his own brother, Tostig, killing him when he came back, with a Viking army, at the Battle of Stamford Bridge.

After William the Conqueror seized the English crown, imprisonment became the norm. It provided something that exile and execution could not: leverage. In 1141 King Stephen was captured and imprisoned at Bristol Castle. That proved a handy bargaining chip for the Empress Matilda and her supporters. When her main ally, Robert of Gloucester, was captured in battle later that year, the two sides could arrange a handover. When, likewise, the empress’s son, Henry II, imprisoned the Scottish king, William the Lion, it was a major coup: William had to pay homage to Henry, officially subordinating Scotland to English overlordship.

Entire countries could be leveraged in this manner. The Scots had to pay a hefty ransom to get their king back; so, too, just 20 years later, did the English for theirs. On his return from the Holy Land, Richard the Lionheart was captured by the Duke of Austria, Leopold V. As with Robert Curthose, the imprisonment of a crusader aroused papal opposition: Celestine III excommunicated Richard’s captor. The real burden, however, fell upon the long-suffering English taxpayer. Leopold handed his royal hostage over to the Holy Roman Emperor, Henry VI, who – after gloating in a letter to Richard’s most determined enemy, Philip Augustus of France – demanded from the English a ransom of 150,000 marks. Leopold hoped to get something else for his trouble: the hand in marriage of Richard’s niece for his son and heir. But Leopold died before the arrival of Eleanor, Fair Maid of Brittany, so the marriage never took place. According to the chronicler Arnold of Lübeck, Leopold’s imprisonment of the English king had been so terrible a sin that it explained why providence had continued to prevent the crusaders from reclaiming the Holy Land.

Thus far in English history, royals were arrested and imprisoned by other royals, either because they were deemed to be a threat or, in Richard’s case, because they could be squeezed for cash. The 13th century saw a new principle develop: that Parliament could, if it so wished, lock up a king who had transgressed some boundary. After his defeat at the Battle of Lewes in 1264, Henry III was held in captivity for 15 months; the baronial junta that seized power, led by Simon de Montfort, was careful to present itself as governing by his legitimate, royal authority. Still, Parliament was often little more than a royal plaything. When Edward IV arrested his brother, the Duke of Clarence, in 1477, ‘all that was envisaged’, in the words of Clarence’s biographer, was a ‘cooling-off period in the Tower’. And yet, by January the following year, Clarence was tried, condemned and executed. This was achieved by a simple act of Parliament, which had been packed with royal placemen and agents of the Woodvilles, the family of Edward’s queen. Elizabeth Woodville’s father had been beheaded due to the conniving of Clarence and Warwick the Kingmaker; they had also hassled her mother, Jacquetta of Luxembourg, by trying her for witchcraft. Clarence’s arrest thus presented Queen Elizabeth with a good opportunity for revenge.

What, then, is a royal to do in captivity? Mary, Queen of Scots spent her long imprisonment cooking up schemes: in the stern judgement of her unsympathetic biographer, Jenny Wormald, this was an ‘altogether characteristic combination of heedless ambition and hopeless political judgement’. Charles IX of France said of her, in 1572, that ‘the poor fool will never cease’ from plotting ‘until she lose her head’ – and so, in 1587, she did. Others turn to poetry. Richard the Lionheart vented his frustrations in a ballad, ‘Ja nus hons pris’ (‘No man who is imprisoned’). According to the 18th-century antiquary Edward Williams, better known by his bardic name Iolo Morganwg, Robert Curthose learned enough Welsh during his captivity at Cardiff Castle to compose some verse in the language (Williams/Morganwg, it must be said, was something of a fabulist).

From the Wars of the Roses to the Glorious Revolution, arrest, imprisonment, and even execution have been occupational hazards for members of the royal family. Since then – in this and so many other respects – Britain has been a comparatively quiet place. There was a close shave in 1809, when the Duke of York was enmired in a scandal that combined dirty passions with dirty money; but the government of the day pushed through a motion in Parliament absolving him of any ‘personal corruption or criminal connivance’. Today, the man formerly known as the Duke of York probably cannot expect from Prime Minister Keir Starmer the clemency that his predecessor got from Spencer Perceval. He can, however, breathe a sigh of relief to live in a country that has forged its principle of the rule of law over the centuries, and still takes nothing to extremes: better, certainly, to be Mr Mountbatten-Windsor than Citizen Louis Capet.