The story of “Princess of Amathar” takes place in the world of Ecos. Notice I said takes place “in”, not “on”, for Ecos is a Dyson Sphere. There is a lot of information on Dyson’s Spheres on the web and quite a bit relates to an episode of Star Trek: The Next Generation. I however set Princess of Amathar in a Dyson Sphere long before that episode was shown, having read about it in a role-playing magazine (for the Traveller RPG).
Ecos is a giant hollow ball, about 180 milliion miles in diameter, with its sun in the center. The people walk around on the inside of this great sphere. Though the Ecosian sun is slightly smaller than ours, you can’t tell because it is slightly closer to the surface of Ecos than ours is to Earth. This vast shell provides a surface area that is billions (with a B) of times larger than the surface area of any normal planet.
Ecos was created ages ago by a race of beings known as the Elder Gods. They also populated Ecos with dozens, perhaps hundreds of alien races, who then developed their own civilizations and societies. No one knows what happened to the Elder Gods, but the many races of Ecos continue to thrive.