Drukgyal Dzong means ‘Fortress of the Victorious Drukpa’. Drukgyel Dzong was believed to have been built in 1646 by Zhabdrung Ngawang Namgyal to commemorate his victory over the Tibetan invaders. Others account that it was Tenzin Drugda, the second Desi, (who was Paro Penlop at the time) who assembled it at the command of Zhabdrung Ngawang Namgyal.

The Drukgyel Dzong structure consists of a tall central building and an adjacent courtyard surrounded by lower buildings. The fort is located on top of a hill with steep cliffs on three sides and a single entrance to ensure that it is not vulnerable to attacks. It is heavily guarded by several watchtowers situated between the entrance and foot of the hill. There also used to be tunnels providing protected passages for people to fetch water from the river below the hill but these tunnels are now sealed.
The ancient fortress served as an important defense base in the region and housed sacred documents until 1951 when it was almost completely destroyed by fire. Unlike the other ancient fortresses in the country, Drukgyel Dzong is the only dzong used for defensive purposes without any religious or administrative functions. It is one of the four principal Taa Dzong (defense fortresses). The other defense fortresses in Bhutan are Gasa Trashi Thongmon Dzong, Haa Damthang Dzong and Lingzhi Dzong.

When the invasions by Tibet finally ended, Drukgyel Dzong became an important location for the traders. Rice was the main export from Bhutan to Tibet, while salt and tea were the main goods imported from Tibet. Caravans would stop by Drukgyel Dzong before going through the Tremo La pass.

The existing ruins and original defense structure of the Drukgyel Dzong are well preserved and protected. The ancient ruins of Drukgyel Dzong is a famous archaeological site in Bhutan. This impressive structure was even featured in the National Geographic magazine in 1914. Despite the fire in 1951, the glory of Drukgyel Dzong remained until today. It is listed as a tentative site in Bhutan's Tentative List for UNESCO inclusion.

In April 2016, to celebrate the birth of the Dragon Prince, Gyalsey His Royal Highness Jigme Namgyel Wangchuck, as well as to commemorate the arrival of Zhabdrung Ngawang Namgyel to Bhutan in 1616 AD and the birth year of Guru Rinpoche, then Prime Minister Tshering Tobgay announced that the dzong will be restored and reinstated to its former glory upon the command of His Majesty King Jigme Khesar. The announcement and ground breaking ceremony took place on 6 February 2016, a day after the Crown Prince was born.

Reference: https://www.drukasia.com/bhutan/paro/drukgyal-dzong/