The Pueblo people are well-known for building cliff-dwellings, apartment-like complexes built from stone, adobe, or mud, into or near cliffs or canyon walls that overlooked open spaces or plazas. These multi-storied complexes could house hundreds of families, which accessed their own apartments, at different heights in the cliff or mud wall, by using ladders. Some apartment complexes were so massive, such as those at Chaco Canyon, New Mexico, that they served as the largest building structures in North American until the 19th century. Some complexes were so large that they included over 700 rooms, although the average complex included about 200 rooms. In addition, Pueblo people built ceremonial rooms called kivas that were circular or (or less often rectangular) rooms used for spiritual purposes. Kivas were originally built underground by ancient Puebloan people, but as they evolved they were more often found above ground. Kivas normally included a fire pit, benches, stone columns known as pilasters, ventilation tunnels and a small hole called a sipapu which was built as a spiritual "doorway" that symbolized the pathway by which the Pueblo ancestors entered the world.