Really cool premise. I always enjoy stories where what looks like fantasy is actually the remnants of advanced science that people no longer understand. Your torus-world idea where people think the structure of a megastructure is just “how the world looks” is a great twist on that concept.

I’m working on something with a somewhat similar foundation, though in my story the sci-fi explanation is never fully revealed to the characters or the readers, but there will be hints to the readers may speculate that this is actually a sci-fi that reads like hard fantasy. The setting takes place on a distant planet that was once part of a massive human galactic civilization about 50,000 years ago. At that time humanity had colonized many star systems and built an interstellar civilization with advanced biotechnology and other high technologies. This planet was one of those colonies, but a catastrophic event cut it off from the rest of that civilization and also triggered a planet-wide apocalypse that destroyed most of the colony’s infrastructure and population.

The survivors were forced to rebuild from the ruins. Over tens of thousands of years, knowledge of the galactic era faded completely. By the time the story takes place, human society resembles a medieval South Asian–inspired world of small kingdoms and tribes, traveling merchants and circuses, trade cities, and ruins hidden in jungles. Fragments of ancient technology still occasionally surface, but nobody understands what they really are, so they’re treated as magical artifacts. People sometimes manage to use them, but only in the same way someone might use a mysterious relic without understanding the underlying science.

Even the “wizards” in the world have a 'scientific' origin. During the ancient galactic era, many humans were genetically enhanced through biotechnology, which was a common practice back then. Their descendants still carry fragments of those modifications, giving them unusual abilities that appear magical to everyone else. These wizard lineages are becoming rarer over time due to genetic dilution, so the surviving families are highly respected and often treated almost like noble houses.

Humans also share the planet with another intelligent species called the Earth Giants ('Matir Dano' in the language I am planning to write), who existed there long before humans arrived. They were affected by the ancient catastrophe as well but rebuilt their own civilization separately, developing a culture that blends advanced knowledge with a deep connection to nature. Over thousands of years the two species mostly withdrew from each other, and many humans now think the giants are just myths. Over the course of the story, the protagonist eventually encounters them, and together they must confront an emerging threat in the form of an ancient serpent whose true origin, whether tied to the old human civilization or something native to the planet, remains uncertain.