Just for fun, wanna spin a setting idea a bit, ...

[Figleaf23]

... post-apocalyptic fantasy.

Elements:

-A beautiful, smart, practically eutopian TL3 society is harshly destroyed by ??.

-Most survivors are reduced to TL1, and TL0 in some places.

-Substantial great architecture like castles, palaces, city walls and aqueducts survived the cataclysm, some stands empty now, some in use by TL1 squatters.

-Advanced knowledge of magic (style/kind as yet to be chosen) was known to the precursors, but has since been almost completely lost.

That's what I've got so far. The floor is open ...

[Jovus]

This seems like you might very easily tie this in to GURPS Banestorm if you were interested.

The PCs are elves right after the cataclysmic casting that devestates Yrth. Most elves are dead, most forests are destroyed...and to make things worse, in the coming years, there are strange new things in the wasted lands, stalking down the remnants of the civilized peoples and the orcs alike.

[RyanW]

It reminds me a bit of Conan (note that my knowledge of Conan comes principally from the Arnie films). I think, principally, the feeling I get that steel would become an almost mystical material.

[William]

The reason most of the great architecture isn't inhabited is the ghosts. The apocalypse shut the road to the afterlife, perhaps to protect it from the repercussions of the war in the mortal realm, or to seal away a great evil. Now, everyone becomes a stable ghost upon death, and it takes the personal intervention of an angel, demon, or similar spirit to haul a soul across the Veil. Obviously, few are holy enough to call upon angels and demons make unreliable guards. Apparently destruction leads to a ghost's reformation near a place of power a few days later.

There are entire cities, manors, and fortresses manned by the various legions of ghosts loyal to powerful mages or ancient warriors. Some are conquest-oriented; others wish to play the role of benevolent ancestors. Many are simply mad. Unaligned spirits are easy prey for the rapidly-increasing population of necromancers, one of the few areas where magic is close to the level of the previous age, so most of the living pledge their (eventual) loyalty to a local Grey Duke. Some living tribes dare to bind and bottle ghosts that attempt to demand loyalty from the living, but such independence is always dangerous.

Desperate quests to find and reopen the gates of Heaven and Hell are common for both the living and the dead, but the magic of gates and planar journeys is nowhere near recovery. If there is a door to the underworld in the mortal realm, it is closed, and must be reopened. Of course, many of the worst of the dead have no desire to face judgment, so these quests are firmly opposed.

[Mailanka]

If you're going to have precursors, you should make good use out of them, and for more than just well-stocked dungeons.

First, there's the mystery of what destroyed civilization. Second, there's lingering magic. The Romans left roads and aqueducts. This civilization might have left behind some powerful (and now possibly dangerous) magical infrastructure. Old, magical guardians, dangerous weapons of mass destruction, magnificent ruins leaking dangerous, mutative power.

Post-apocalyptic settings often has a sense of grandeur: "We made this. We were capable of doing this once." There's also a sense of danger. Technology is both magnificent, and often the cause of our demise, and so the protagonists are reaching for a double-edged sword, wanting to rekindle the fires of civilization, but wary that those fires might spread out of control.

Take the elements of sci-fi post-apocalyptic stuff and rewrite them in a fantasy miliu. The crystal guardian of the Eternal Sanctum has gone mad and rules over his too-perfect city, completely devoid of human life while its golems patrols the streets, the broken ones performing the same tasks over and over in haunting pantomime while primitives eke out a life in the tunnels below, running from the hunting eyes of golems.

Elsewhere, the empty knights of the Empire of the Fallen Star take to the roads every night when the moon hides its face from the earth, hunting for the last bearer of the imperial bloodline, of which there are several, each with a potent and dangerous magical heritage locked within.

Orcs (or something more creative) have broken free of their creators, torn themselves out of the magical labratories that created them and have claimed ruins as their own, trying to come to grips with their complete lack of heritage and their cruel magical nature, "genetic experiments" trying to find their freedom.

TL 1 is also a time of heroes. You could emphasize that by creating distinct lineages of heroes or magic weapons. Such a setting is ideal for uncovering the glories and sins of ones ancestors, and forging a new destiny.

In fact, you could draw quite some inspiration from Exalted

[ericthered]

You could have a world that had powerful and common magic that suddenly broke down and no longer works. In one instant existing economic and military systems are obsoleted. Men who could rule as Gods before are now powerless. transportation falls, food production falls, the military is no longer competent, and chaos ensues, destroying civilization. (I recently read Elantris)

The great quest for this world is "why did the magic break, and how can we fix it?" perhaps it was a grand act of blasphemy or a lost battle in in the afterlife. Perhaps it was an experiment that is draining all the magical energy in the world. Maybe the language of magic has been changed to a different accent.

[samd6]

I find really hard to crash a TL3 society down to TL1 in such a way that they couldn't recover within a generation or two.

I'd personally go for at least TL3+3^ before the disaster.

A large chunk of population (at least 30%, possibly up to 90%) needs to die off fast, leaving fewer survivors.

[Sindri]

With lower tech apocalypses the trouble is that most people can pretty much just keep on doing the things they did before since not much is required to support their industry. A lot of the technological knowledge is something that everyone would know. Like we would not lose the germ theory of disease just because we were knocked back to TL3 in general. Also most places are self sufficient or reasonably close to areas they can get food from.

I could easily see a loss in magical technology. I suppose if you kill a really risky percentage of the population you might go down a TL or two. Alternatively perhaps a magical curse or something makes farming a lot harder so societies have to give up some experts to survive.

[William]

Originally Posted by samd6

I find really hard to crash a TL3 society down to TL1 in such a way that they couldn't recover within a generation or two.

I'd personally go for at least TL3+3^ before the disaster.

A large chunk of population (at least 30%, possibly up to 90%) needs to die off fast, leaving fewer survivors.

A lot of folks probably did die off. I think the biggest thing that needs to happen to go from TL3 to TL1 is for the secret of iron to be lost. The blacksmiths, geometers, and alchemists may have been the focus of the apocalypse -- the craft magicians whose artifices disturbed the world so greatly that it brought down the event that shook the foundations of the earth. That gives you a good reason for a lot of knowledge to vanish -- the knowledgeable were the ones that died. (There may be scrolls and inscriptions remaining, but if most of the survivors were the illiterate masses, writing is a nearly-lost mystery as well.)

ETA: An old idea I had for a fantasy setting -- planehopping PCs didn't spend much time there -- was a Bronze Age society where dwarves kept the secret of iron. Very powerful magic there, at least as the humans and the fey saw it. Oooh, did the elves ever hate that dwarven iron.

[Mathulhu]

Invasion of Demons who can easily be killed with iron or steel.
Once the population centres have collapsed from debauchery the demons go home, taking their play things (the people who opposed them) with them.