Story started with the abduction of a number of inhabitants of a planet by technologically superior aliens. I think the abductors and abductees were all human. They are transported by spaceships to an artificial mega-structure, a galaxy sized ellipsoidal object with an aperture at one end. The inner wall of the mega-structure is inhabited and apart from the aperture are isolated from the rest of the universe. The abductees are destined fight in some war or conflict in this enclosed habitat.
There are many stars floating in the vastness of the structure's interior which is light years in extent. The stars have been engineered and placed to provide energy for the galactic sized inner living surface. The vastness of the structure is described in detail. It blew my mind when I read this and I still regard it as one of the most original books I ever read.