Python
"I have glimpsed, like shadows behind the realities, the dim shapes and outlines of valleys, forests, mountains and lakes that are not as they are today, but as they were in that dim yesterday -- have even sensed, rather than glimpsed, the purple towers of forgotten Python shimmering like figures of mist in the dusk." -- Robert E. Howard: "The Hour of the Dragon"
So-called City of Purple Towers, ancient capital of the evil empire of Acheron, its extent was ten times that of Luxur, Old Stygia's greatest city. Its wealth was beyond imagining, and its mages and priests the most powerful the world has ever seen. Legends state that its obelisks were "high enough to pierce the moon". Its royal palace had a gate called the "Ruby Gate". By Conan's time it had been extinct for 3,000 years.
Although Buccaneer infers that Python was located in the Shem-Stygia region because of its appearance on an ancient map, this is certainly an error on the part of the mapmaker. Acheron occupied the land that later comprised Aquilonia and Nemedia. Its capital must be sought in that region. The great antiquity and sophistication of the Nemedian civilization, its secure territorial position which enabled it to resist later invasions, and the conjuring up of the sorcerer Xaltotun within Nemedia all seem to point to the location of vanished Python in that country. Xaltotun was formerly high priest of Set in Python before it was wiped out by barbarians. Although he died in the South, he was resurrected by Nemedians who must have retained racial memories of his ancient prowess. We have placed Python tentatively on the plain north of Hanumar in an area that is now open grazing land. (A strong case could also be made for placing Python in Aquilonia, however.)
At the time Acheron flourished, a river that subsequently changed its course presumably made the area suitable for a large population. Hyborian invaders from the North would have viewed the purple towers on the fruited plain as a sitting duck and eventually razed Python to a mound of rubble.
References: Hyborian Age I, Hawks over Shem, Flame Knife, Conan the Buccaneer, Hour of the Dragon, Conan the Rogue, Conan and the Treasure of Python.