The Time Lords originally discovered the Yssgaroth's universe during early experiments in making black holes. These experiments accidentally created ruptures in the space-time continuum, releasing the Yssgaroth. Other openings to the Yssgaroth's universe could be found in 1800s London, near Stonehenge, and on the unnamed planet in the Althosian system. (PROSE: The Pit)
The Book of the War speculated that the Yssgaroth were simply side-effects of the collision between the Spiral Yssgaroth and the Spiral Politic, and the Yssgaroth were simply the Great Houses and lesser species' projections of their fears onto areas of hostile anti-structure. (PROSE: The Book of the War)
The terrain appeared to be mostly desert. The universe had unusual proportions and distances, making travel even more difficult. There were many settlements of suffering humans, who were generally used as slave labour. It was the origin of Dream B.
Species including humans, Ogrons, Terileptils, the Galk, and Bapputchin had been accidentally brought to the Yssgaroth's universe. Other species, such as the Cun, were either native or just unknown to the Seventh Doctor. Still others, like the Hunters, could naturally travel to the Yssgaroth's universe. (PROSE: The Pit)
During the Last Great Time War, the Time Lords, at the command of Cardinal Ollistra, released the Great Vampires from their universe to fight off a fleet of Dalek saucers. Despite the War Doctor's protests, Councilor Voltrix proceeded to open a rift, allowing them through. (COMIC: The Bidding War)