Ring world

A ring world was like a Dyson sphere, but shaped like a ring instead of being completely spherical. Safeplace was a ring world about 32,000 light-years from Mars. A segment of the Key to Time took the form of a castle where the Valdigians negotiated on Safeplace. (AUDIO: The Judgement of Isskar)

Behind the scenes
The concept was invented and used in 's novel, as well as subsequent books in that series.