The city of Tây Sơn is 40 kilometers north-west of the center of Hanoi, in the former province of Hà Tây, which belongs to the Greater Hanoi since 2008. Because of its strategic location on the Red River on the outskirts of the capital Sơn Tây was in his history in important military outpost, especially against the invasions from the north. Large parts of the well-preserved citadel was built in 1822 in the third year of the reign of the Nguyễn emperor Minh Mang, on the grounds of the two villages Thuần Nghệ and May Trai.
The square fort with a length of about 450 meters is now in the heart of the city about two miles from the Red River. She was completely surrounded by a five meter wall with loopholes and a three-meter deep and 20 meter wide moat. On each side was a gate guarded by cannons, which was reached via a stone bridge, the Cửa Hậu was located on the north, the Cửa Tiền in the south, the Cửa Hữu in the the west and the Cửa Tả in the east. The north gate and the adjacent wall sections have been restored some time ago. The West and the South Gate have also been preserved, while the gate in the east is destroyed. The entrance to the fortress is possible only through the North or the South Gate.
Inside the area is the 18-meter-high flag tower, which has also been renovated recently and a palace that served alternately as the administrative headquarters, warehouse and troop accommodation. The citadel was in the 70s and 80s of the 19th Century a center of resistance against the French colonial army, but was conquered by the French on the 16th December 1883 after more than one year of resistance. The Governor-General of French Indochina let put it on the list of historical monuments of Tonkin in 1924. Since 1994, the citadel of Sơn Tây belongs to the National Heritage of Vietnam.
A visit to Sơn Tây can connect very well with a journey into the past to the community of Đường Lâm which is quite close to the town.