Dark Circus

The Dark Circus, as Kingpin called it, was the dimension, (PROSE: The Greatest Show in the Galaxy) the true time-space of the Gods of Ragnarok. It was an arena of grey stone slabs with inscriptions on them that included carvings of moons and statues of clowns and other figures. While the Gods could create a bridge for people to enter the Circus, the main entrance was through a well on the planet Segonax, possibly indicating that the Circus did exist on the planet, albeit in another dimension.

In ancient times, the Gods lured gladiators to the Dark Circus and fed on the entertainment, afterwards waiting frustrated and hungry for Kingpin to show up. The Seventh Doctor entered their realm and entertained them briefly, before using their own medallion to reflect their energy; this presumably destroyed the Gods, who began to crumble along with the Dark Circus. (TV: The Greatest Show in the Galaxy)
 * Since the Gods and the Dark Circus were made of stone and the Gods had other forms outside of their realm, it's possible they could not fully leave their own dimension, just as the Celestial Toymaker, another Old One, could never fully leave his realm.

Behind the scenes
In the reference book The Legend Continues, the Dark Circus is described as "the past of their temple."