Tartaria: Are the cool old buildings in your town the repurposed remains of a lost civilization?

[Warpy]

4. The latter, I suspect

Experimental archaeology has shown the people of the time with the technology of the time could and did build this stuff.

There are still some mysteries, like the megalithic foundation stones the Incas built on. We know how the stones were fitted, but not how they got there.

We also have no idea how the Neanderthals managed to extract birch bark pitch on an industrial scale to make their adhesive. We know what's in the final product, we know what temperature the bark needs to be held in an oxygen poor environment, we just don't know how they managed to do it.