To help explain tons of dungeons and ruins in my campaign world (link in my sig if you're interested), I created a "layered" history of fallen empires. Essentially there are four time periods wherein different flavors of things happened that can lead to "remnants" of adventure sites. They are:
1) pre-history - before the First Miraborian Empire, there was a huge span of time in which the gods created the varied races, they spread over the world, orcs came into existence, a god fell, new gods arrived, dragons became epically powerful and then slid into ruin, etc...
2) First Miraborian Empire; this (world spanning?) empire had many components: powerful and prevalent giant races; sky-castles (a reputed "sky-realm"); many great cities of men, elves and dwarves; many races lived together in seeming peace and cooperation at the peak of the era; several epic wars for which great war machines were produced; powerful magics far beyond what can be produced today (this was an era of 1-30 lvl PCs; my preferred campaign style now is Epic 6th). A period of perhaps a thousand years.
3) first interregnum between empires; during this period the sky-realms were cut off from the earth, and perhaps vanished; the giants and (to a lesser extent) dwarves were deeply affected by the "rock plague" which caused 90% of the giants to die off, and forced their retreat from the known world in all but tiny numbers. Magic was deeply changed during this time; no-one now living knows how or why, but magic nearly vanished from the world, lingering only in tiny secret pockets. Most of the gods fled or were greatly weakened at this time, as well. A period of several hundred, perhaps a thousand years.
4) Second Miraborian Empire: new nations arose, but in a much smaller region. The Second Empire only ever truly ruled one continent, and part of another at it's peak, which lasted about 400 years. Total evolution from end of the first interregnum to final collapse, about 800 years. Dwarves and Elves both suffered setbacks during this time, as humankind dominated. Elven and Dwarven secret kingdoms either established or re-sealed. Magic grew again, but was never able to surpass the bounds that lie upon it today. New gods rose, or the old ones awoke again? Of course, over time, this empire dug deep, raised great cities, and eventually grew old. Slave revolts, civil wars, magical catastrophes occurred, and eventually the empire collapsed again, shattered into a thousand small pieces.
5) the current era; 400 years after the Second Empire's "official" fall, small nations are beginning to grow again. Humans, elves, dwarves and other races are establishing their realms, and beginning to meet in friendship and rivalry. Much exploration of the past is ongoing, and expansion for the future. It's a good time to be an adventurer!
This has given me a campaign world that has lasted, evolved, grown and twisted over the past 25+ years. It has enough depth to handle just about anything I want to throw at it, yet has room to take almost any concept and squeeze it in somewhere. There's some realism, but not too much. I'm far more interested in versimilitude than "accuracy".
