Looming over Cusco  to the north are the ruins of Sacsayhuamán (7 a.m.–6 p.m., daily), a hilltop fortress with three ramparts of zigzag walls that run for nearly 300 meters on its north side. The largest stones—nearly 8.5 meters high and 361 tons, according to historian John Hemming—were placed at the apex of the walls to strengthen them.
Every Inca citizen had to spend a few months of the year working on public works, and the Inca used this tremendous reserve of labor to move the stones, using log sleds and levers. But even engineers have a hard time understanding how the Incas fitted these huge stones so perfectly together.
Only the largest stones of Sacsayhuamán remain. Up until the 1930s, builders arrived at Sacsayhuamán to cart away the precut stone of this apparently limitless quarry, so it is difficult to appreciate how impregnable Sacsayhuamán must have been. Three towers once crowned the top of Sacsayhuamán, and two of their foundations are visible.
During Manco Inca’s great rebellion, the Spaniards managed to establish a base on the opposing hill and spent two days charging across the plain on horseback and attempting to scale the defensive walls. On the first day, one of the stones fired by the Inca slingshots struck Juan Pizarro, Francisco’s younger brother, who died that night.
On the evening of the second day, the Spaniards launched a surprise attack with ladders and successfully forced the Inca into the three stone towers. As the Spaniards massacred the estimated 1,500 soldiers trapped inside, many Inca preferred to leap to their deaths from the high tower. The next morning, condors feasted on the dead bodies, and this grisly image is emblazoned on Cusco ’s coat-of-arms.
These days the flat fields outside of Sacsayhuamán, where the Inti Raymi  culminates each June, is a peaceful place to stroll. In the mornings, Cusco  residents come here to jog or do yoga on the grassy lawn, which is considerably larger than a soccer field. A huge trapezoidal door leads up a walkway to the top of the ruins, which is a marvelous place to bring wine and watch the sun setting over Cusco . Because many tourists come here in the evening, guards are posted to put visitors at ease as dusk falls.
If you have time, visit the top of Rodadero hill, where the Spaniards based themselves during their assault on Sacsayhuamán. There is a rock outcrop on top, beautifully carved with sacred steps. Sacsayhuamán is a steep, two-kilometer walk from Cusco  or a 10-minute taxi ride (US$2). Taxis wait in the parking lot for the return trip to Cusco.