The Old World, also known as the Undercity, was the name given to the first city built by the Eternal Empire on the planet Zakuul.
The Old World was located at the surface of the planet Zakuul, at the foot of the Spire, and above Breaktown. The Undercity was rather different from the Spire whose inhabitants lived in a near-utopia. As the Eternal Empire security forces were virtually absent from the area, alternative economy prospered in the Old World. Largely controlled by criminals, the area provided unsavoury services that were absent in the Spire. The Undercity had therefore acquired a nefarious reputation and became a place to visit for the privileged Zakuulans who were fascinated by suffering but did not really know much about it. Nevertheless, despite its nefarious reputation, the Undercity remained a quite safe place, especially compared to Breaktown. Ironically, Spire denizens continuously brag about venturing in the Old World as a feat while most of them visited Oldtown in their life.
The Old World seen from the surface of Zakuul
The Old World was built by Emperor Valkorion amid the swamps of the planet Zakuul. Despite the construction of the city was a real success, it was not grand enough for the Emperor, so a second metropolis—the Spire—was built atop the Old World. Sections of the Old World around the Spire's base remained intact and inhabited.
During the War against Zakuul, Emperor Arcann allowed the fanatical Heralds of Zildrog to take over the Old World in order to keep order in area in place of the Knights of Zakuul.
During that conflict, the Alliance Commander visited the Old World twice: in the company of Senya Tirall while looking for the Lady of Sorrows in 3631 BBY, and in the company of Kaliyo Djannis while planting explosives at the power junctions in preparation for their assault on the Overwatch in 3630 BBY.