The most distinct and important structure in Bhutan, Dzongs (fortresses) are architectural masterpieces that dotted Bhutan. Each Dzongs are built on a site chosen for religious significance.
Bhutanese dzong architecture reached its zenith in the 17th century after the arrival of the great lama Zhabdrung Ngawang Namgyal. Each of the dzongs site were based on omens and visions. The dzongs were well sited with regard to their function as defensive fortresses. Hilltops or mountain spurs were usually chosen as a site to build dzongs, and if dzongs were built on the side of a valley wall, a smaller dzong or a watchtower is typically built directly uphill from the main dzong whose main purpose would be to keep the slope clear of attackers who might otherwise shoot downward into the courtyard of the main dzong below. The Dzongs are part of a network that defended the Kingdom against frequent invasions by Tibetans in the 17th century. Dzongs have stone foundations and walls of sand and clay bricks, and wooden beams are skilfully cut to fit most dexterously.
By tradition, dzongs are constructed without the use of architectural plans hence there is no blueprints of any of the dzongs. Instead a high lama who established each dimension by means of spiritual inspiration gave construction directions. Traditional Bhutanese architecture did not use any nail;s or iron bars. Dzongs serve as the religious, military, administrative and social centres of their districts and are often the site of an annual tsechu or other religious festivals.