Stand at over 12,000 feet looking up at a wall of massive limestone boulders, some weighing up to 200 tons, fitted together so seamlessly you cannot slide a sheet of paper between them. No mortar, no iron tools, no modern machinery. Your brain simply struggles to process how human hands achieved it.

This deep dive into Sacsayhuaman, the colossal Inca citadel above Cusco, Peru, unpacks the unfathomable scale of Inca engineering genius. Drawing on 16th-century chronicles and modern studies, we explore how the fortress was built, why it survives earthquakes that level rigid walls, and the tragic irony of how it was partially dismantled. It matters because it forces us to ask what our own civilization will leave behind in 500 years.

• Cusco was laid out in the shape of a puma, with Sacsayhuaman built as the head, and chronicler Cieza d ...