The famous twelve angled Inca stone. The fact that this stone has been cut at twelve different points to give stability to the structure demonstrates the skill and intelligence of the Inca’s minds. You can’t even fit a sheet of paper between the stones.
The Incas realised that Cusco was an earthquake prone area and took precautions to give a strong foundation to the walls that withstood the test of time. The structures of the Incas were so well made that the Spanish conquerors used them as a base for their own buildings instead of demolishing them and didn’t even know themselves how to replicate these methods.