Theoryland of the Wheel of Time

One was evidence of discovery of steel, the first manufacturer of steel, which was discovered—we found countless places where the first smith discovered how to make steel, or at least a smith discovered how to make steel, and by all the evidence, no one else in that area had known how to make steel before him—but of course when he could make steel, his weapons were much better than anybody else's. He had provided steel swords to use against bronze, or iron, and...wow, he had sort of a magic sword here, didn't he? And he sure as hell didn't teach anybody else how to do it, so from the time that men began discovering steel, and the secret began dying with them, to the time when steel began to be manufactured semi-widely, was about a thousand years.

The first time that gunpowder—that we can find evidence of gunpowder being used as a weapon—was in China, when the inhabitants of a besieged city made huge fireworks and dropped them over the wall onto soldiers trying to climb ladders, siege ladders up over the walls of the Chinese city. It's not a very efficient way to use gunpowder, but what's interesting is that it was something over a thousand years after gunpowder, by the evidence, had been discovered in China, and for all of that time, it had been used for nothing more than making fireworks, firecrackers, just...that was it. That was the whole use. So, we have a world where there are no guns because nobody knows how to make gunpowder, except for this guild, and they're not going to put the secret around.