YA about a girl finding an underground city or underground prison

I remember it's a historical setting and the teen girl lives in a castle or estate . She finds a hidden door in one of the rooms that leads underground and finds a underground city or prison and falls in love with a teen boy there . She ends up helping him escape the city and the people from above are trying to keep her from helping them .

Sounds vaguely like Incarceron by Catherine Fisher, if some of the details were distorted by time/memory. It's science fiction/fantasy in which the female lead lives in an artificially maintained 17th-century setting. She learns to communicate with a boy who's spent his whole life in a mysterious prison. The prison isn't technically underground (its location is a spoiler so I don't want to say too much!), but access to it is through a secret door in the palace where she lives.

Thanks so much! That's the book (: After all these years having pieces of the book in my head and looking everywhere I'm so grateful to you and this website for finally finding it. I can't wait to read it once more !

Great! Glad you found it.