I was thinking in terms of ancient Greece like the GP mentioned. That said, Romans invented and pioneered concrete, but they didn't really understand how or why it worked, it was just centuries of excellent trial and error. They also had problems replicating it out of base materials other than volcanic ash.
Now that said I have my doubts that there are that many people on earth who could build better concrete for a given application than the Romans with no store to go buy pure materials from, I'm certainly not one of them.
That's also why I didn't talk about modern steel -- In my head I vaguely understand that there's a chain from copper to wire to a rotor/stator to electrolysis to oxygen gas to the Bessemer process, but I'd be amazed if there's anyone on earth who could bootstrap it in one lifetime, even with an ancient king's resources.
In general, as someone else mentioned in the context of agriculture, the low-hanging fruit are probably innovations that aren't especially complicated or require possibly uncommon elements and other material. Stirrups for example in Europe. Also mostly abstract (but possibly useful) scientific and medical knowledge.
Anything that requires a long technology tree to implement effectively is going to be hard.
Now that said I have my doubts that there are that many people on earth who could build better concrete for a given application than the Romans with no store to go buy pure materials from, I'm certainly not one of them.
That's also why I didn't talk about modern steel -- In my head I vaguely understand that there's a chain from copper to wire to a rotor/stator to electrolysis to oxygen gas to the Bessemer process, but I'd be amazed if there's anyone on earth who could bootstrap it in one lifetime, even with an ancient king's resources.