Manga FL refuses engagement to Prince

Question

Read this manga approximately a year ago on mangafire.

Characters:

Main character was a female lead of this manga, absolutely no memory how she looked but I distinctly remember that she hated a prince who she was engaged to.

Prince might have killed or threatened to kill her in her past life.

Magic tower friend? Main character had a male friend from the magic tower, I'm quite sure he had white hair.

Story:

The main character has been isekaid or reincarnated. The world might have been from a book she was reading.

I remember a fountain where MC was waiting/passing by and she met a girl who was the main heroine? or something like that. The MC liked her.

After the aforementioned fountain scene they started to make way to the school opening ceremony probably and before/after the ceremony MC stopped because she spotted the Prince. They argued about the engagement and she was quite rude to this Prince.

Quite a bit later MC joins the magic tower to keep the Prince from killing her? as Royal family and the magic tower seem to have a pact of no harming eachother.

Later the King requires her to stay in the castle for sometime to force the engaged/marriage onwards. Her family seems to not care about her feelings and sends some servants to keep an eye on her during the forced visit.

She absolutely goes nuts in the castle breaking things and being as annoying as humanly possible. Even the magic tower old wizard boss assists her in destroying some stuff from the castle roof.

Later King gives up and lets her go.

Answer 1

This is I'm No Heroine!, aka I Will Politely Decline the Male Lead!. It is licensed by Lehzin.

A girl obsessed with a novel suddenly finds herself transported into the thick of the plot. The problem is, she has taken on the role of the villain in the story. And she knows how this ends…it’s off with her head! Maybe she’ll just let this play out and wake up back in reality… But after a few too many deaths to no avail, it’s finally time to go off-script. She's going to have a little fun with this tale. She’s made up her mind…to live!

The main character is a girl that reincarnated in the world of a novel she had read, as the villainess. She decides to play out the events of the book and dies after being condemned by the prince and heroine of the book. She comes back to life at the start of her reincarnation.

After dying another time she decides to go off the rails and not get involved with her fiance, the prince. She still meets the heroine, and as a fan of the book likes her, so she becomes friends with her instead of harassing her this time. They first meet in this life at the fountain in front of the school.

She also befriends a knight and a magician while at the school; the latter allows her to enroll in the mage tower to avoid royal interference.

The scene with her being stuck in the castle is around chapter 38. Her constant abuse has made her fiance interested in her, and the king is also intrigued by her advances with magic. They conspire to get her trapped in the castle and refuse to let her leave, which her family is supportive of. She does not take it well and causes as much mayhem as she can.