Square Tower House in Mesa Verde National Park, Colorado, USA

© lightphoto/Getty Image

Castle in the cliffs. Square Tower House, Mesa Verde National Park, USA

Welcome to the impressive Square Tower House in Mesa Verde National Park in Colorado, USA. Square Tower House in Colorado is one of many cliff dwellings constructed by the Ancestral Puebloans, an ancient Native American culture. So it’s a fitting homepage image for Unesco's International Day for Monuments and Sites.

This 27ft (8m) structure is notable for its kiva, a round, sunken room used for ceremonies. Unlike most of the ancient kivas found in the region, this one has an intact original clay roof supported by wooden beams.

Ancestral Puebloans lived in what is now known as the Four Corners region, where the borders of Colorado, Utah, Arizona and New Mexico meet at a single point. It is thought that they appeared here a little more than 3,000 years ago and built cliff dwellings for defensive reasons, as factions competed for scarce resources amid a prolonged drought. It's thought that the changing climate eventually drove them from the cliff dwellings around 1300 CE, just a century or so after construction began. Today, many of the cliff dwellings have been restored and visitors can view the homes and kivas of the builders.