Perhaps the greatest of all the Incas' architectural achievements, Sacsayhuamán sits proudly on a hill overlooking the empire's erstwhile capital city of Cuzco. A massive walled complex attributed to Pachacuti, its bastions, towers, temples and aqueducts exhibit such stunningly masterful stonemasonry that when the Spanish beheld its grandeur they refused to believe that it was the work of mortals. Even today, after suffering partial destruction at the hands of the Spanish, Sacsayhuamán still takes the breath away of all who visit and is a living testament to both the architectural expertise and administrative mastery of Tawantinsuyu.

While better known as a military fortress, early chroniclers referred to Sacsayhuamán as a temple called the "Royal House of the Sun" devoted to sun worship. It was also fundamental in the overall design of the Incas' glorious capital city, given Cuzco was conceived as a gigantic puma with Sacsayhuamán as its head. The complex's Quechua name is usually translated as "satisfied falcon" but there are a few alternative theories such as "royal eagle" and "speckled head".

How the Incas accomplished the herculean task of creating Sacsayhuamán eludes scientists to this day. While they have not resorted to the conquistadors' theory that demons were the responsible party, researchers still have difficulty grasping how the Incas transported gargantuan rocks weighing nearly 200 tons and fit them together with such precision that not even a blade of grass could fit through the mortarless joints. According to some estimates, 20,000 to 30,000 men would have been required to complete the job and it would have taken about 60 years. The Incas' structures are also remarkable for their earthquake resistance. Built leaning inward with perfectly interlocking stones of irregular shapes, the walls of Sacsayhuamán have survived calamitous earthquakes that razed Spanish structures to the ground.

Among Sacsayhuamán's most outstanding structures are its outer tiered walls built parallel to one another in a zigzag pattern. Reaching 20 feet high and stretching up to 1,312 feet long, the walls contain colossal boulders that dwarf the tallest of men. The Spanish removed the stones in the upper portion of the walls in order to construct churches in Cuzco, however, which is why the walls are intact only up to a certain height. In the enclosed area three fortified towers once stood, forming a triangle. The central tower, known as Muyucmarca ("circular place"), was cylindrical while the other two, Paucarmarca and Sallacmarca, were rectangular. Muyucmarca consists of three concentric circular stone walls connected by three channels supposedly designed to conduct water into the tower, which was purportedly a clean water reservoir. Each of these towers was destroyed by the Spanish and today only foundations remain. The Incas also constructed an impressive network of underground passageways that connected Sacsayhuamán with other Inca structures in Cuzco.

One of the most decisive battles in the conquistadors' final defeat of the Incas took place at Sacsayhuamán in 1536. When Manco Inca rose up in rebellion against the Spanish, who had formed a settlement in Cuzco, he occupied Sacsayhuamán and sent forces down to attack the Spanish from the hilltop fortress. Although the Spanish lacked reinforcements and were very nearly overcome, they somehow managed to break through Manco's defenses and into the fortress, where they exterminated the native troops in two days' time. However, the heroism of one Inca nobleman would not go unnoticed. Cahuide, as he was known, occupied Muyucmarca, the last tower to remain in Inca hands, defending it from each and every enemy soldier who attempted to seize it. When it became clear that victory would go to the Spanish, Cahuide took his life by jumping from the top of the tower, preserving his own honor and dignity from the hands of his enemy.

Today, Inti Raymi celebrations are held near the ruins each year on June 24. Sacsayhuamán is located approximately half a mile from Cuzco's San Cristóbal church and is easily reached by foot or bicycle, but taxis and buses can also take you there.