Keden Qan was the first Royal Engineer of the Kingdom of Lether. Keden served the kingdom from its founding to its sixth decade. During the planning phase of the settlement building plan, Keden made a thorough survey of the land, and found that the three extant Jhag towers were actually a part of a larger complex, something that did not quite make sense due to their wandering and individualistic nature. This meant that the site of the city of Letheras was actually a pre-dispersion site for the Jaghut, a place where the Jaghut, or some of them, lived in a community before they split up. An alternative explanation given by Keden was that the site belonged to a loyal Jaghut family that stuck with each other.
However, beyond the Jaghut complex, were other ruins that belonged to another race. One had been, and still was, an Azath House, and encompassed an area almost the same as Letheras at the time of Reaper's Gale. The evidence included still existing foundation walls, plazas and concourses, not all of it of Human, or even Jaghut or Tarthenal design, all built in a way that was not designed to encompass the above mentioned sites. Based on all this evidence, Keden Qan came up with the conclusion that Letheras had been built on the site of an earlier city, one that predated even the Azath House and the Jaghut towers.
Keden Qan was either unable or unwilling to identify the species that built these earlier buildings. However, Qan's analysis did reveal that the denizens of this city frantically tried to add insulation, most probably due climate change. Presumably, they failed.
The above information was derived from Janath Anar, an imprisoned scholar, who tried to occupy herself with mental exercises during her imprisonment. One exercise was recalling the lectures she gave at the Academy of Imperial Learning. Due to the extreme distress she was in, some information from her lecture might have been recalled incorrectly.