uncovered traces of cities from the 10th- to 12th-century Kidan period. The most significant excavated city was Hatun Hot, founded in 944. Another significant Kidan city was Bars-Hot in the Kherlen River valley, which covered an area of . The city was surrounded with mud walls which are now thick and high. Yurts The yurt, traditional dwelling of Mongolian nomads, is a circular structure supported by a collapsible wooden frame and covered with wool felt. In Mongolian, a yurt is known as a (). During the 12th and 13th centuries, (yurts on carts) were built for rulers. Large iron bushings for cartwheel axles were found during excavations at Karakorum. Axle length was over , and the cart was pulled by 22 oxen. Such s are mentioned in the Secret History of the Mongols. Medieval yurt camps were commonly arranged in a ('circle'), with the leader's yurt in the centre. s were replaced by an (neighbourhood) arrangement in the 13th and 14th centuries during the Mongol Khanate and the end of internal strife. After the 15th-century disintegration of the khanate, the was the basic layout of monasteries (which were initially mobile). Another type of monastery layout, (following the Tibetan arrangement) was used during the 16th and 17th centuries when Buddhism was re-introduced to the region. As monasteries and camps developed into towns and cities, their names retained the word (for example, Niislel Huree and Zasagtu Khaan-u Huree). Roofs originally had steeper slopes, with a rim around the center opening to allow smoke from central, open fires to vent. During the 18th and 19th centuries, enclosed stoves with chimneys () were introduced; this permitted a simpler design, with a lower silhouette. Another relatively-recent development is an additional layer of canvas for rain protection. The organization and furnishings of the interior space mirror family roles and spiritual concepts. Each cardinal direction is significant, and the door always faces south. Herders use the sun's position in the crown of the yurt as a sundial. Yurts have been used in Central Asia for thousands of years. In Mongolia they have influenced other architectural forms, particularly temples. Between 30 and 40 percent of the population live in yurts, many in city suburbs. Tents Tents played a role in the development of Mongolian architecture, and these temporary shelters were frequently used under pastoral conditions. Tents were erected for Naadam, feasts and other gatherings. is a small tent,