Sacsayhuaman is a walled complex located on the northern slopes of the city of Cusco. It is often described as a fortress since it is enclosed by three slopes and has a series of parallel walls that zigzag and would expose the flanks of invaders. However, archaeologists are unsure of the purpose of Sacsayhuaman and it's possible that it may have been a temple devoted to the worship of the sun or used to form the head of the Puma that Sacsayhuaman and Cusco make when viewed from above.
The walls at Sacsayhuaman are constructed with huge boulders that fit together like a jigsaw without any mortar and is remarkably resistant to earthquakes. The larger boulders can exceed 100 tons are fit together so precisely that it's impossible to fit a sheet of paper between the rocks. Unfortunately, knowledge of the construction methods have been lost and today researchers can only speculate as to how Sacsayhuaman was built.