SOLVED. SciFi starts as Fantasy, with a face-covering symbiotic parasite and a planet divided by great barriers

[Nicholas]

I read this book in 2015-2019, and I can't remember any details about the author or title. I've tried Googling with combinations of many search terms and have had no luck in finding it.

Starts as Fantasy with main character in the mountains in a fantasy medieval-ish civilization. Eventually, they go downriver to a large city and somehow discover they have a power to pass through walls. The main character's civilization is surrounded by a great barrier, and they travel through that barrier to discover multiple other 'worlds' on the same planet, each separated by massive barriers.

One world has a parasite which covers the host's face and is somewhat symbiotic (eventually used to the group's benefit, or is very relevant to story of the world I think?).

One world contains people first thought by the main character to be "yahoos" (I remember this specifically from the book), since they are first seen naked and in the branches of trees. These people are actually the most advanced civilization on the planet, and are the only ones which were allowed to develop flight.

I'm not sure why exactly this planet divided into multiple (I think seven?) civilizations exists (although its purpose is revealed at the end of the book), but it is a separate planet from a more advanced Earth.

At the end of the book the characters must defend against some alien threat.

[Nachelle]

This sounds like Orson Scott Card's "Pathfinder" series.

[Nicholas]

Yes, that's it! Thanks for the help!

[Kris]

Nicholas, did you read the first book Pathfinder by Orson Scott Card - or more books in the "Pathfinder" series: https://www.goodreads.com/series/5135... ?

[Nachelle]

You're welcome!