The flag tower, or Cot Co, was a later addition to the Hanoi Citadel complex, built by Emperor Gia Long with the help of French engineers in 1805. The tower, which affords excellent views of the surrounding area for anyone with the energy to climb its winding stone steps, currently stands on the grounds of the Military Museum. The tower is 33.4 metres tall and offers a grand view of what's left of the Hanoi Citadel.
Unlike many buildings and structures in Hanoi, Cot Co was not razed during the French administration of 1896. Instead, it was used as a military post. Comprised of three tiers and a pyramid-shaped tower, Cot Co is illuminated by 36 flower-shaped and six fan-shaped windows. Note that you may have difficulty snapping a graceful picture of the tower, as the Army Museum next door tends to block most of the good views.