Machu Picchu is tangible evidence of the urban Inca Empire at the peak of its power and achievement—a citadel of cut stone fit together without mortar so tightly that its cracks still can’t be penetrated by a knife blade.
The complex of palaces and plazas, temples and homes may have been built as a ceremonial site, a military stronghold, or a retreat for ruling elites—its dramatic location is certainly well suited for any of those purposes. The ruins lie on a high ridge, surrounded on three sides by the windy, turbulent Urubamba River some 2,000 feet (610 meters) below.
Scholars are still striving to uncover clues to the mysteries hidden here high in the eastern slopes of the Andes, covered with tropical forests of the upper Amazon Basin. Machu Picchu appears to lie at the center of a network of related sites and trails—and many landmarks both man-made and mountainous appear to align with astronomical events like the solstice sunset. The Inca had no written language, so they left no record of why they built the site or how they used it before it was abandoned in the early 16th century.