It was from here that the present royal family emerged as the most powerful force at the beginning of the last century. Protected from invaders by an impenetrable valley, Trongsa Dzong is an impregnable fortress. It is built on many levels into the side of the hill and can be seen from every approach to Trongsa heralding its strength as a defensive stronghold. The Dzong was originally built in 1648 by Shabdrung Ngawang Namgyal, which was later extended by its powerful Penlops, and the sixteenth Desi Sonam Lhundub. The town's dzong is particularly impressive and is often described as a dragon flying over mountain peaks. Constructed in 1644, it served as the original seat of power of the House of Wangchuck before it became the ruling dynasty of Bhutan in 1907.