opposed to the rammed earth more prevalent in the west. Doors of Bhutanese homes usually have a tongue-and-groove design, hinged on a pair of wooden pegs. Generally, smaller windows are built into lower floors, with larger windows on upper levels to add to structural soundness. Windows are most often decorated with a curved trefoil (horzhing) motif. Sloping, wooden-shingled roofs are pitched but lack rain gutters; there is a growing trend toward metal shingles for their durability. Traditional roofs are adorned by a wooden cornice. There is usually a large space left between the roof and walls for air to pass. Interior stairs are cut from single trunks when possible. Dzong architecture Bhutanese dzong architecture reached its zenith in the 17th century under the leadership of the great lama Ngawang Namgyal, the Zhabdrung Rinpoche. The Zhabdrung relied on visions and omens to site each of the dzongs. Modern military strategists would observe that the dzongs are well-sited with regard to their function as defensive fortresses. Dzongs were frequently built on a hilltop or mountain spur, or adjacent to important streams. Dzongs comprise heavy masonry curtain walls surrounding one or more courtyards. The rooms inside the dzong are typically allocated half to administrative function (such as the office of the penlop or governor), and half to religious function, primarily the temple and housing for monks. This division between administrative and religious functions reflects the idealized duality of power between the religious and administrative branches of government. Religious architecture Buddhist temples (lakhang) in Bhutan are often relatively simple single-story structures surrounding a courtyard. Most also feature high thresholds. They are often adorned with a red stripe along the upper walls, and gilded copper roofs. There is sometimes an antechamber at the entry. Internal walls and congregation halls of Bhutanese temples are decorated with painted and appliqué frescoes and murals. Religious themes predominate, especially the life of Buddha, the legends of Guru Padmasambhava, and protective deities. Monasteries (gonpa, goenpa) follow two architectural traditions: cluster and dzong. Cluster types seem to present the oldest tradition of monastic architecture in Bhutan, in which one or two temples is surrounded by clusters of housing for its monks. Chortens, receptacles of worship akin to stupas, dot the land. Pilgrims and locals circumambulate chortens in order to gain merit. Larger Bhutanese domed chorten are hewn of stone and whitewashed in the Nepalese style. Smaller flared chortens in Tibetan style