All Purpose Muling Device
Arkarianscribe
Well that was... rather unhelpful.
I'm open to the idea of curses, just not the changing curses. Someone once recommended that she has some sort of influence over men, either repulsion or infatuation. And that's something that I'm looking into.
But even that begs the question of how she actually gets to the tower. Do her parents send her, and if so where do they get the idea to do so?
I'm setting up to help you, I just had questions first. Don't be a d**k to someone trying to help you. It would help if you answered why she has to not be in contact with anyone ever.
The idea to send someone to such a prison would likely have come from tradition, and it wouldn't matter how the original one was thought of in that case. It's just "how it's done." Alternatively, you could go for it being the only instance of such a thing, and the thought process could be "well, she's our daughter and we can't bear to kill her, but we need to keep her away and out of sight."
If you don't want to do curses, consider having her be the sister of a monarch who feels threatened by her. There are plenty of instances where siblings of monarchs were imprisoned for such reasons (and not executed, because that would be poor form). Feeling threatened could come from her being a more popular figure, at least with a certain portion of the population. Imprisoning her there would keep her out of the way, but alive and so not rousing the anger of the populace.
Frankly, though, this is one of the worst ways to actually imprison someone. If you think too hard into it, you should see that going a different route is the most realistic. I'm not saying that to convince not to do it, but rather to allow more of a handwave of realism.
I don't want there to be contact with a captor because that involves so many more complications, as well as strays from the classic outline, which I'm doing in numerous other ways and would like to avoid another.
I do really like you idea of a threatened monarch. That was something that I had never considered. It could be an evil queen trying to get rid of her or a princess... and perhaps an old witch or something offers to take the princess away and lock her away to where she'll never return... That is a very excellent idea.