According to the National Park Service, the great structures of Mesa Verde were built by a people now known as the Ancient Puebloans who first occupied the region about 1400 years ago. Initially they lived on the mesa tops in subterranean pithouses. About 750 AD they began building complex multi-storied houses above ground. By about 1000 AD they developed sophisticated stone masonry techniques, using sandstone, with structures two or three stories high in units of 50 or more. Traditional hunting and gathering was supplemented by agriculture that focused on maize production, apparently without irrigation.

By the Classic Period (1100-1300) the population at Mesa Verde expanded to several thousand people, initially living in nucleated villages still located on the mesa tops as seen in the Far View sites today. After 1200 much of the population shifted off the mesa into compact communities located in large hillside alcoves as seen in 'cliff dwellings' such as Spruce Tree House, Cliff Palace, Balcony House, and Long House. These famous cliff dwelling sites that we see today were constructed between the 1190s and the 1270s.

By 1300 most of the people of Mesa Verde had migrated away and Mesa Verde was largely unpopulated. There's still much debate among archaeologists about why the Ancient Puebloans abandoned the region -- though prolonged drought, over-population and depletion of resources, and warfare have all been suggested. The Ancient Puebloans are seen as the ancestors of many of the modern-day Pueblo peoples of New Mexico as well as the Hopi in Arizona.

Note that there has been considerable stabilization and restoration of the ruins at the major cliff dwelling sites. Long House on Wetherill Mesa is the most intact of the major cliff dwelling ruins with the least amount of reconstruction.