Dzongs serve as the religious, military, administrative, and social centers of their district. They are often the site of an annual tsechu or religious festival.
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.
Bhutanese dzong architecture reached its zenith in the 17th century under the leadership of Ngawang Namgyal, the 1st 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. Wangdue Phodrang dzong, for instance, is set upon a spur overlooking the confluence of the Sankosh (Puna Tsang) and Tang Rivers, thus blocking any attacks by southern invaders who attempted to use a river route to bypass the trackless slopes of the middle Himalayas in attacking central Bhutan. Drukgyel Dzong at the head of the Paro valley guards the traditional Tibetaninvasion path over the passes of the high Himalayas.
Dzongs were frequently built on a hilltop or mountain spur. If the dzong is built on the side of a valley wall, a smaller dzong or watchtower is typically built directly uphill from the main dzong with the purpose of keeping the slope clear of attackers who might otherwise shoot downward into the courtyard of the main dzong below (see image at head of article).
Punakha Dzong is distinctive in that it is sited on a relatively flat spit of land at the confluence of the Mo and Pho Rivers. The rivers surround the dzong on three sides, providing protection from attack. This siting proved inauspicious, however, when in 1994 a glacial lake 90 kilometers upstream burst through its ice dam to cause a massive flood on the Pho Chhu, damaging the dzong and taking 23 lives.