These structures at Ollantaytambo were under construction when the Inca empire fell. Standing there, we couldn't fathom how people possibly could have manipulated those massive stones, artistically fusing them together, as if they were building granite countertops with modern tools. And this was granite, too, a hard igneous rock. Each stone weighed thousands of pounds. They'd all been moved, cut (somehow) and set into place without the use of steel or iron or mortar.
We'd had spent years as stonemasons, working for the National Park Service in a similar style (sort of.) So it seemed like we should have been able to figure out how they did it, especially since the site was still halfway built, exposed and vulnerable. But there is no way to understand Ollantaytambo.