Flag Tower is located on Dien Bien Phu street, Ba Dinh district, Hanoi; near Ba Dinh Square. Hanoi Flag Tower, also called Cot Co, is one of the rare architectural works in Hanoi . It was used by French troops as an observation tower and communication station between command headquarters and adjacent military posts.
It was built in 1812 and is composed of 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 to the sunlight." The tower receives sunlight through 36 flower-shaped and six fan-shaped windows.