Lena Dunham and Bella Ramsey Share Trailer for Medieval Coming-of-Age Comedy Catherine Called Birdy

Lena Dunham always knew she was going to adapt the beloved children's book Catherine Called Birdy into a film.

"From the time that I knew I was going to make movies, I thought I want to make that movie," the Girls creator, 36, tells PEOPLE of bringing Karen Cushman's popular 1994 book of the same name to the screen. "When I had the opportunity right after Girls to kind of think about what I wanted to do next, I spoke to the author and optioned the rights."

The medieval comedy, set in 1290s England, premieres at the Toronto International Film Festival in early September before opening in theaters and arriving on Amazon Prime Video — and on Wednesday, the first trailer for the film dropped showing Game of Thrones breakout Bella Ramsey as the titular Lady Catherine.

Dunham tells PEOPLE it was actually her dad who first brought Ramsay, 18, to her attention in the HBO juggernaut series.

RELATED: Lena Dunham Jokes Costar Jon Bernthal Should Teach a Class on 'How to Be a Man in a Sex Scene'

Amazon Prime

"My father had told me, 'There is this girl on Game of Thrones. You are not going to believe what she's doing.' He was like, 'I can't even describe to the energy this kid is bringing.' And so I had loved her there."

Like Game of Thrones' Lyanna Mormont, Ramsey's Lady Catherine (known as Birdy) is strong-willed and wise beyond her years. When her financially destitute father (Fleabag's Andrew Scott) attempts to marry her off to a wealthy suitor, Birdy puts up a fight, challenging her parents and the norms of the society in which she's raised.

"I always grew up being quite bold, I suppose," Ramsay tells PEOPLE. "I was never intimidated by anything really — especially not boys my age. So I guess that's why I tend to really enjoy playing these roles. What appealed to me, especially about Birdy, was watching a girl grow up in a time that wasn't really designed for her. She's struggling against this cage that the world has kind of built for her. It's just so interesting to see her kind of grow up and learn how to exist in the cage, but not let it limit her. I think she's funny and wacky and chaotic, and I love her."

Alex Bailey/Amazon Prime Video

Dunham says despite its period setting, Catherine Called Birdy is as timely as ever.

"It spoke to me back when I was 10 because of my own feelings of complexity around puberty," she shares. "And it speaks to me now because I think we're always going through many adolescences over and over as we grow."

"It's interesting how much of that also now echoes what it means to be woman in the United States now and what it means to be born into a female body, born with a female reproductive system, the expectations and the terrors that come with that," Dunham adds. "And this film is very, very much about bodily autonomy and about trying to claim a safe space for yourself in a culture that enacts a particular kind of violence on you, just because of the organs that you're born with, the sex organs that you're born with. I always am marveling at the way that sort of history is far away, but not that far away."

Catherine Called Birdy opens in select theaters Sept. 23 and arrives on Amazon Prime Video Oct. 7.