Since the earliest drafts of what has evolved into Frozen Skies I've always been rather keen on having a fallen ancient civilization, partially because old ruins and artefacts always help provide a plot hook for adventures. The Ancient Terrans seemed a natural fit as a name for this lost civilization, if anything it helps muddy the waters as as history is concerned in the world of Darmonica.
The inspiration for the Ancient Terrans seems from the old PC game Arcanum: Of Steamworks And Magick Obscura, there was a ruined city called Vendigroth that had been the heart of a highly technological culture and had been destroyed. Certainly the part about its high technology being destroyed along with the city greatly appealed to me, especially the aspect of the world's technology level gradually recovering over time but still a far cry from what had existed. Also the fall of the Ancient Terrans would help with regards to working a calender and a timeline, a reference point for dates and stuff if you will.
In Frozen Skies there is the Monolith that stands as the most visual remnant of the Ancient Terrans, its ruined spire sitting forlornly in the heart of the impossibly flat and circular Chillwynd Marches. In addition there are various old ruins scattered throughout the continent of Alyeska and beyond, some of which have been adapted as the foundations for modern settlements or became snow-bound piles of rubble. I had envisioned Ancient Terran artefacts sparking a 'relic rush' when the well-monied started taking an interest in them to grace the display cabinets of their billiard rooms and later on curious inventors sought to unlock the mechanisms of some of the devices that the ice gave up. But not all of these artefacts are harmless.
Course I'm going with something that caused the downfall of what could be described as Darmonica's technological equivalent of our own world's Roman Empire, though for now I intend to keep that something vague and let players and their GMs determine what happened to the Anicent Terrans in their game.