Fortunately for medieval haircare budgets, Rapunzel and her sisters in the folk tale type "Maiden in the Tower" are primarily that--folk tales. But there are some interesting specific things to look at anyway. Or at least, interesting if you like killing abusive fathers with lightning.

First: while lonesome towers imprisoning beautiful virgins might not have been scattered across later medieval Europe, one story about one tower sure was. St. Barbara was one of the most popular saints in western Christianity. She was never a real person (or at least, there's no historical evidence for her). But her legend made her one of the select group of saints known as the virgin martyrs. These saints--most of whom were apocryphal like Barbara--were Christian teenagers (always female) or young women in the days of the early Church and pagan-Roman persecution, who refused sex and/or marriage for religious reasons, stood up for their faith, and were violently killed.

The thing about medieval virgin martyr legends, especially the apocryphal saints, is that they're awesome. Katherine of Alexandria defeats fifty pagan philosophers in a head-to-head debate. Margaret of Antioch gets eaten by a dragon, then kills it by splitting open its stomach from the inside and bursting out. And Barbara is our maiden in the tower.

As Barbara's hagiography has it, her father Dioscorus built a tower and confined his beautiful daughter there to keep men from seeing her before marriage. But whereas we might see a prison, the situation didn't seem to bother Barbara all that much--she was far more upset when her father tried to present her with suitors to choose from, since she'd chosen her Prince Charming already and he was Christ.

One interesting thing about Barbara's hagiography as opposed to other virgin martyrs is that her father doesn't seem to have a problem with her turning down marriage. His reaction is to...build another building (in some versions, for her use specifically) and go out of town for awhile. When Dioscorus does return, though, he finds out that Barbara had the builders alter his plans for the non-tower building. She insisted that it have three windows instead of two, to reflect the Christian idea of God as a Trinity. He was so angry about her Christian devotion that he nearly killed her on the spot, but virgin martyrs don't get off that easy she was miraculously transported out of the building and onto a mountainside.

...But virgin martyrs also don't get off without being, you know, martyrs, so one of the shepherds betrays her back to her dad, for his trouble God turns him to stone and his sheep to locusts.

Then Barbara ends up in real prison, and then violently dead. And afterwards, God smites her father with lightning--and then smites the ashes left behind so that not a trace remains.

But while virgin martyr hagiographies have a strong tinge of romance to them (amidst the gory torture and death, of course), actual tower or castle captivity for medieval women would not be told as a tale of tragic quasi-adventure. Lords forced female family members into service as hostages in treaty negotiations or other agreements. Sometimes this came in the guise of marriage or betrothal, or "planned" versions thereof, but sometimes it came down to handing over a prisoner. And the resulting picture of the woman's life also varied greatly.

Consider Constance (d.1201) and Eleanor of Brittany (1184-1241), mother and daughter. Constance was the daughter of Conrad II of Brittany, who made the grievous mistake of owning land that English king Henry II wanted closer control over. Conrad was "persuaded" to send his daughter Constance to Henry's court as a future wife for Henry's son Geoffrey--when Constance was four. This wasn't a case of two families making a betrothal or arrangement for the future; Constance was sent to the other court.

However, Constance did seem to have a relatively normal life for a twelfth-century English princess. Annette Parks remarks that we have no evidence of whether or not Constance helped in running the duchy--just significant absences by her husband, in which medieval noblewomen often stepped up...and, oh yeah, a long-burning conflict between Geoffrey and his father.

It was Eleanor who was in trouble.

First, there was the failed attempt to make her a hostage. (No, really). When the Holy Roman Emperor imprisoned the King of England after the Third Crusade, Heinrich did so intending to extract a significant ransom for Richard. Part of that ransom was hostages sent from England to Germany to act as, essentially, a guarantee that England would pay everything off eventually. Eleanor was to be one of those hostages, in the guise of marriage to Leopold of Austria's heir. But Leopold had the gall to die when Eleanor was only halfway across Europe, so she was sent back to England.

And say goodbye to Richard the Lionheart and hello to King John. It didn't take John much to see that as the granddaughter of Henry II and Eleanor of Aquitaine, Eleanor of Brittany had a strong claim for her descendants (if not in herself) to a whole lot of territory--including a future claim to the throne of England, a.k.a. John's throne). It also seems that it didn't take John very much to defeat a rebellion by Eleanor's older brother--and one of the results was John's physical custody of Eleanor.

And so just like the apocryphal Barbara's apocryphal father built a tower to keep her away from men and marriage outside of his control, John thrust Eleanor into a castle away from men and marriage. As Parks writes:

Once John had captured Eleanor, he and his heirs had little choice but to keep her. Even when she had passed her child-bearing years and was no longer a threat to produce claim-holding heirs...mere possession of her person brought with it implicit and explicit claims to land, position, and power that required careful management.

One wonders what Eleanor thought of thirty-nine years of "careful management": allowed servants and the space to go horseback riding, but never again leaving the castle where she was shut up--and with no Prince Charming coming to save her. Deity or otherwise.

And then, let's go to the place where history and hagiography meet back on the other side: the granddaughters of English king Henry I this time, and why we shouldn't brush off stories of hostages as guests with restrictions. Henry was trying to pacify relations between two of his nobles involved in what super-comprehensive 12th century chronicler Orderic Vitalis noted as one of the few blood-feuds (Latin talio) in his knowledge. He had Ralph Harenc send his son to Eustache of Breteuil (Henry's son-in-law, by the way), and in return Eustache sent to Ralph his daughters.

Eustache, unfortunately, was interested primarily in using the situation to lash out at Ralph. He had Ralph's son's eyes gouged out...and sent to the boy's father as a "gift." In return? Ralph asked Henry for permission to blind and cut off the noses of his own hostages, Eustache's daughters...Henry's own granddaughters. Henry granted it.

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If you'd like to read more about virgin martyrs (specifically the hungry dragon part) OR saving princesses, you can pre-order my book How to Slay a Dragon: A Fantasy Hero's Guide to the Real Middle Ages. Learn how to survive your next dungeon crawl or self-insert fantasy fic with evidence from actual medieval history...and a handful of really terrible pickup lines.

It was not unheard of. Towers were certainly a common place of confinement, and sometimes the people being confined were noblewomen.

When dealing with prisoners, there were two options for where in a castle to put someone: somewhere high or somewhere low. You don't want them to have easy ground floor access and definitely not window access on the ground floor. This made your average room in a castle rather unsuitable, as they were designed for day to day use, so they almost always had windows and multiple entrances. Unless a lord decided to construct a dedicated building to house prisoners, towers were appealing choices. Many castle towers were segmented into floors with the bottom floor either serving as a dungeon (in some cases being dug down into the earth to prevent escape) or as storage space that could easily be turned into a dungeon if the need arose. The towers of medieval castles typically lacked direct access to the bottom level, with the entrance on the first floor and an internal ladder down to the bottom level of the tower. With the prisoner surrounded by stone or earth, the bottom layer of a castle tower was theoretically a fantastic space to keep prisoners.

However, keeping prisoners in the bottom floor of a tower had certain pros and cons. One big problem was that if the tower was badly built then the prisoner could escape. In one instance, a prisoner simply dug underneath the tower. Another time, a tower collapsed because a prisoner used the stuff in the storage area (where he was being held) to smash his way out. It wasn't ideal. So some lords decided that rather than use the bottom level of the tower, they would use the top level. We have many recorded instances of castle towers being used as prisons. For example, a French royal counsellor and abbot called Sugerius (writing in the middle of the twelfth century) records that a certain Anselm de Garlande was imprisoned in the tower of his own castle after it was taken by Hugh de Crécy, and that Louis VII ordered captives to be held in the tower of Gournay Castle. It is also recorded that Geoffrey of Anjou kept his captives chained in a dark room at the top of a castle tower. We know that it was common to hold people in castle towers. It is also architecturally obvious where a castle tower served as a dedicated prison - they have no windows. Over time, it seems to have become much more common to hold prisoners in the top level of a castle rather than the bottom level in castles where there was no other dedicated building for prisoners. Some such towers became famous for their utility as prisons even if that wasn't their primary purpose much of the time, most obviously the Tower of London. Despite having windows, its height, location, and the tower's garrison made it a good prison. Gruffudd ap Llywelyn ap Iorwerth, a Welsh prince of the early thirteenth century, died when he attempted to escape the Tower of London via a rope made of sheets and cloth only to find that he was a bit too heavy. The manuscript illustrations are quite funny though.

So the idea of locking people up in castle towers was certainly popular and commonly put into practise. But you might notice that all the examples I've given so far have been men. It was rare for noblewomen to be confined at all, let alone locked in a castle tower. Duchess Eleanor of Aquitaine, famed for her strong personality, patronage of early chivalric literature, as well as her astounding personal charm and beauty, was detained by King Henry II of England after she supported their children in a rebellion against Henry. King Philip IV of France locked up his allegedly promiscuous daughters in towers to preserve their social standing. The teenage noblewoman Isabel de Clare, Earless of Pembroke, was kept in the Tower of London until her marriage to William Marshal in 1189 (a practise called wardship), but apparently in some of the nicer rooms rather than the prison bits. This was allegedly to keep her safe from abduction, but really to use her as a bargaining chip or reward for royal service. For more on this kind of imprisonment, see u/sunagainstgold's answer.

That being said, there were times where noblewomen were confined to towers and kept in terrible conditions, though this was rare. Margaret of Burgundy, wife of King Louis X of France, was suspected of adultery and locked in a castle tower until the conditions destroyed her health and she died (some think she was poisoned, but confinement certainly wasn't doing her any favours). King Philip IV of France locked up his daughters-in-law, also for adultery, though one got a pardon later. Betrayal of this nature was seen as legitimate grounds to lock a noblewoman in the upper chamber of a castle tower.

Although medieval noblewomen were supposed to be treated decently, on some occasions the treatment of these women could be shocking. It was believed that King Stephen of England, prior to his rule, had imprisoned the daughter of a knight he disliked and allowed his men to rape her. When this was discovered there was a local revolt. When a disgraced nobleman named Thomas of Marle was excommunicated and hunted down in 1130, one of the charges against him was that he had chained and raped a countess. A noblewoman locked up by their own family could expect to at least not be raped, but this was not the case for many other imprisoned women, even those of high status, who did not have the security of a familial connection to their captor. It may be interesting to note that although the common poetic presentation of a damsel locked in a tower generally has them miserable but kept in decent conditions, perhaps like Philip IV's daughters, it is a depiction that gestures toward a far grimmer and violent reality.

So yes, noblewomen were sometimes kept in towers. Sometimes this was a punishment for a betrayal of some kind, usually adultery. Sometimes it was indirect retribution or just outright cruelty.

Follow up, how common was establishing a system of government based on strange women lying in ponds distributing swords in some some farcical aquatic ceremony?

But, seriously, are there any real life examples of folk tale-like unusual means of usurping a king or replacing one when a blood lineage couldn't be kept going?