Circus is one of the Strange Stars' great wonders. No one knows who built the megastructure (it may even predate the Archaic Oikumene and be pre-human), but whoever it was had mastered technology beyond the reach of current civilization. It's gigantic ring has a radius of 1.9 million km and a width of about 1000 km, giving it a habitable surface area roughly 20 times that of Old Earth.
It's rotational period 90ks, and it's tilted so that it's inhabitants experience roughly earth-like night and day. The 90ks day of Circus is the standard for the 350 day Circus Kilosecond-Standard Year calendar, which seemingly was designed to simulate the rotational and orbital periods of Old Terra, while eliminating some of the natural variables that make such calendars resort to less than whole numbers.
It's open to space, but centrifugal force and an upper "mesh" of radiation filtering nanites hold a breathable atmosphere in.
In a system bordered by The Zuran Expanse, Alliance Space, Instrumentality Space, Circus has long been a center of trade. Its ruins attest to several stages of colonization by the human phyle. Modern, non-wilderness or waste sections of the great ring are a crazy quilt of petty kingdoms, communes, and experimental societies. These are the "zones"--or at least partial zones. The minimum size required of a political body for the term to be used is the subject of controversy.
The most famous area of Circus is actually a free city--actually a megapolis or ecumenopolis--without a single name. It's most often called Interzone, though it's vast spaceport-adjacent tourist area is known as the Strip. Interzone's boundaries are vague, but including all of it's favelas and industrial parks, it covers an an area only a little less than the surface area of Sol IV (Mars).