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 as another dimension.
In ancient times, the Gods lured gladiators to the Dark Circus and fed on the entertainment, afterwards waited frustrated and hungry for Kingpin to show up. The Seventh Doctor entered their realm and entertained them briefly, before using a medallion to reflect their energy. He presumably destroyed the Gods, who began to crumble along with the Dark Circus. (TV: The Greatest Show in the Galaxy)