So... I did a little more casual research on Wikipedia about the Literary Conan the Barbarian the movie was based off of. (And yes, I know I should always take Wikipedia with a grain of salt. But it's not a bad jumping off point if you follow the citations.)
It turns out the author, Robert E. Howard was starting from the same basic idea I was. He invented an era called the Hyborian Age. Essentially a period of time where there was just enough technology to be interesting, but conveniently before people started keeping records in earnest. There is also a great Cataclysm from which civilization is recovering from. In the case if the Hyborian age, Atlantis disappearing. (In Sumerian myth, the great cataclysm is a great flood, probably the same one from the Hebrew bible. The Semites and the Sumerians were practically neighbors.)
I'm starting to piece together a list of ingredients for a fictional Adventure world. Though, to be fair, I'm just stealing from the same pockets as authors before me. Adventure, it seems, needs to be out in some sort of frontier. In Wild West stories, Adventure happens where settlers arrived before law and order did. In Pirate tales, on the high seas where trade routes overstretch the power of Navies to control them. In Post-Apocalyptic tales, it happens after the bombs fall, and the survivors are left to pick from the rubble. In Sumerian Legend, Adventure happens after the flood destroyed the world, but before Kings started making Empires.
Before I got too far into researching Conan, I noticed another thing: Authors spend a lot of time building mythical history. It's not enough that Kingdom A hates Kingdom B's guts. You have to know why. (Or at least allude to why, throughout the story.) There also have to be heros long gone, who people make allusions to.
My ding against Howard's synthetic world is that it falls back onto a lot of existing legend. And that the level of technology he describes simply did not exist in the 36,000bc to 10,000bc range as described. Howard gets away with it by ascribing a lot of the technology as being leftovers from fall of Atlantis. And conveniently we haven't found Atlantis because it was destroyed.
Very 1920s, but I can't get away with that sort of thing today. We have another century of data from Archeology. And while the answer of human origins still has plenty of mysteries, the development of metal working technology is not one of them. Here is a rough (of the top of my head) timeline of major events era in human history
As we go further back in time, dates get more and more approximate. But it's clear that even if Atlantis existed, and existed 10,000 years ago, Iron working didn't come around until 6800 years later. Steel has actually been around as long as Iron, the trick was how to reliably make steel, which could only be done in small quantities until about 150 years ago. Also interesting to note is that even though the Iron age occurred after the Bronze age, Iron tools are not much of a utility improvement over Bronze implements. The difference comes down to what it takes to manufacture them. For Bronze you need two ores that are not found near one another. For Iron you need just one ore. And Iron ore is much more common.
Iron requires more energy to work. Energy means fuel, and ancient Mesopotamia had to import all of its fuel. The soil in the fertile crescent was great for crops, not forests. Given that the seat of learning for humanity was focused there, Bronze would have been the preferred metal to work with. Tin may be expensive, but when you have to ship in trees all of the way from Turkey and Lebanon, the fuel were more expensive.
The switch to Iron occurs but when trade routes were disrupted around 3200 years ago by a collapse of several major empires, civilizations lost their ready supply of Tin. They had to adapt. Many had more convenient supplies of fuel than the ancient Mesopotamians, and thus the switch over to Iron working. 50 years later when humanity was back on its feet again, there really was no incentive to switch back to bronze. By that point the technological challenges to iron working were already overcome, and the economies of scale made iron working much cheaper. And most development was in places of the world other than the neighborhood around Sumer.
And now we come to something I had never learned before. First that there was a general collapse of civilization at the end of the late bronze age. Second, that period lead to a Dark age in Greece. Greece would rise again, giving us the like of Alexander the Great, 1000 years later. When they started writing again, centuries later it was with a different system.
But for many civilizations, that period of time was it. They were no more. In fact over the span of 50 years virtually every city major city in the Eastern Mediteranian was destroyed by warfare. (And no, not by Iron weapons.) At least in the case of the Roman Empire's fall, Barbarian kingdoms stepped up and took over. The period at the end of the Iron Age was more like the setting for Fallout. Cities are gone. Literacy disappears. Formerly fertile regions are barren. Bands of raiders loot and pillage in the absence of law and order.
Actually, if someone were to pick a period of time that most resembled what we imagine to be the events of Conan the Barbarian, that would be it. You have iron working. There would have been a great deal of riddles with Steel production. There were collapsing Kingdoms, and cults gaining political power. And there would have been complete and utter chaos and upheaval.
And of this Third Ur dynasty period that I'm working on right now doesn't work out for some other reason... at least I know I have a fallback.