Situated on Dien Bien Phu Street, Ba Dinh District, Hanoi; near Ba Dinh Square, Hanoi Flag Tower can be regarded as one of the symbols of a peaceful and beautiful Hanoi capital. Hanoi Flag Tower (called Cot Co), which was finished in 1812 with the help from French engineers, is a later addition to the Hanoi Citadel. Although the Hanoi Citadel covers one square kilometers, it is still much smaller than previous citadels built on the same site. Nonetheless it has walls with are 4.4 meters high. Cot Co is 33.4 meters tall. It was built on the southern end of the citadel, much of which was destroyed towards the end of the 19th century.

Hanoi Flag Tower comprises three platforms and a tower. The words Nghenh Huc, meaning "to welcome dawn's sunlight," are inscribed on the eastern door. The western door bears the two words Hoi Quang, meaning "to reflect light,"and the southern door, Huong Minh, meaning "directed tothe sunlight."  The tower receives sunlight through 36 flower-shaped and six fan-shaped windows.

During Vietnam War, Hanoi Flag Tower was used by French troops as an observation tower and communication station between command headquarters and adjacent military posts. It becomes a historical vestige which links the past to the present inside the complex Thang Long Citadel 1000 years.

How to go to Cot Co Flag Tower: Using the Hoan Kiem Lake as the starting point, walk toward the West along Hang Khay, next Trang Thi. Then you reach a split in the road; the right branch is Dien Bien Phu Street. Take that street, crossing the railway track and keeping on walking until you see the Army Museum on your right and the triangular park with the statue of Lenin on your left. Hanoi Flag Tower (Cot Co) is just behind the Army Museum.