Why do you only find one in towers and only find the other in labranths? What makes it right to sacrifice one and wrong to sacrifice the other?
This all sounds like a contemporary gaming or media thing. In mythology and fairy tales Princesses are confined to towers only in rare instances and for particular reasons, usually involving a curse or a spell and for the princess's own protection. Otherwise, they appear in a variety of guises. Some of them take an active role in helping the heroes perform their tasks, and in that way choose their own husbands. These princesses often possess magical powers. And to win them and half (or all) the kingdom, was not always because they were the daughters of kings, but sometimes because they embodied the sovereignty of the land, and were tutelary goddesses.
To kill Medusa because she was a deadly danger makes sense. Certainly not a sacrifice. She seems to have been able to move about freely, nothing about a labyrinth involved. What happened to her sisters, the other two Gorgons, I don't know. I haven't seen any stories about them, except that they were immortal unlike Medusa, so there wouldn't have been any use sending heroes after them anyway. One could imagine any stories one liked about what ultimately happened to them, but they wouldn't be "canon" or, um ... set in stone.
The sacrifice of a maiden princess, to a sea serpent or dragon or other monster, or to the gods, refers back to the religious and ritual role of royalty in primitive religions. The King might be sacrificed, too, but by the priests or by his successor. We hear stories of princesses being rescued. The stories of princesses who were
not rescued and who were simply killed as a matter of course would not be worth remembering.
"Ho hum, another princess sacrificed." Though in the case of Iphigenia, which was under extraordinary circumstances, there were repercussions that led to further repercussions. But in some stories the goddess rescues her, leaving a deer to be sacrificed in her place, so there is a bit of muddle there.
at least one famous one in a labyrinth (while spinning thread – not a particularly princesslike thing to do. You have people for that.
Ariadne gave Theseus the thread which allowed him to find his way out of the labyrinth, but there is nothing to indicate that she entered the labyrinth herself (except ... maybe ... in Mary Renault). As for spinning, yes, princesses and queens did do that, and weave, too. In the days when kings were little more than tribal chieftains (which were the days when the myths that were recorded or remembered arose), every female who was not dedicated to some other task (in the kitchen for instance) had to spin and weave, or the household went without the clothing they needed. Besides, spinning and weaving play an important role in the feminine mysteries and have a ritual value, too. And let us not forget the Fates, spinning our destinies, and
snip cutting our lives short. In later stories, the religious aspect gives way to magic. If there is spinning or weaving in a fairy tale, you can bet there is magic involved, and sometimes three old women to stand in for the Fates and act as mentors to the heroine.
In the traditional stories, Princesses often
did things, instead of always languishing in towers waiting to be rescued, but they managed to accomplish what they wanted to accomplish without taking on a masculine role and taking up swords.
So, yes, the princess might be a prize to be awarded, but even then she sometimes rigged the contest. She might also be a priestess, or a goddess. And for all practical purposes (putting aside her interesting origins) the Gorgon is simply a dangerous monster.
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