What happens when a medieval world of swords and steel is invaded and forcefully introduced to a System of levels and magic?

It was just another ball.Princess Cassandra thought she could deal with it. She just had to pretend for half an hour to actually care about the attending nobles before her father would let her leave, as usual.

That is, until the sky started glowing blue and strange text appeared in front of her face, mentioning something about a System.

Even that she could deal with.

But being kidnapped by planar invaders from some planet called Earth after a literal glowing heart flies through a portal in front of her and disappears in her chest?

That might be a little too much.

A couple of notes:

There will not be any sexual content or romance in this story at all.

This story is a medieval LitRPG Apocalypse. It begins when a System initializes in a medieval world that doesn't already have magic. The main protagonist, Cassandra de Luna, is a very laidback person. Especially for a princess. She is extremely adaptable and takes things as they are, but will also fight for what she wants when she actually wants something.

The content warnings are mostly so that I have freedom to do that in the future chapters. I normally don't put all that much swearing into my stories, or traumatizing content. Although there does tend to be some gore from time to time in my writing.

1 chapter a week.

The link to book 1 is in the author's note on the first chapter of book 1.