The Anasazi culture are the ancestors of the present Pueblo people and are credited for being skillful builders of the cliff dwellings between the 11th and 14th centuries. Cliff dwellings were not simply occupied caves and openings, but rather skillfully constructed structures with walls, walkways, and roofs built into the canyon walls and on the flat-top mesas. The present four corners area of U.S. Southwest was the predominant location of the cliff dwelling Anasazi. The best-preserved examples of those dwellings are in parks such as Chaco Culture National Historical Park , Mesa Verde National Park , Hovenweep National Monument , Bandelier National Monument , and Canyon de Chelly National Monument. To access fertile river valleys and abundant water supplies, cliff dwellings were principally on the tributaries of the Rio Grande River . Not only were the Anasazi craftsman builders, they were experts at irrigating their crops. For defense, access to the cliff homes was very difficult. Cliff dwellings were often organized in large communities such as the Mesa Verde National Park , in Colorado , where there are more than 300 dwellings.