As the 19th century turned into the 20th century and archeologists started to press deeper in to the jungles of Central and South America and into the deserts of Mesopotamia and India, they began to encounter clear evidence of many civilizations that had attained some level of technological greatness. You look around at these places and you see the living descendants of these people living without the incredibly sophisticated caliber of technology that their forebears had invented. I think it's a very haunting, stark memento mori for a representative of any civilization.
And then another related quote:
The rise and fall of civilization is this inevitable process, to which we must all eventually succumb. Nobody's going got be more haunted by that thinking than a parvenu, an ariviste who's kind of new to it all. The person who's most worried about losing everything is the person who's had it the least amount of time.
This can, perhaps, inform not only the PCs we create, but also the higher level NPCs -- the movers and shakers of the land -- who are perhaps closer to the former glory, are perhaps long-lived survivors of that fall, are perhaps seeking to recapture that former greatness.
And the seeds to that greatness may lie in some forgotten ruin. Somewhere.