The planning of the Citadel took place in 2 years 1803 and 1804. At the time, Emperor Gia Long and Madarin Nguyen Van Yen had a field trip, planned architecture and construction ground.
Compared to Phu Xuan Citadel in late Nguyen Lord reign and Tay Son reign (1786-1801), the Citadel was further enlarged. As planned on the design, the Citadel is located on the 2 distributaries on left bank of the Perfume river. This area was home to Kim Long river, Bach Yen river and 8 villages founded for centuries ago, namely Phu Xuan, Van Xuan, Dien Thai, An Van, An Hoa, An My, An Bao and The Lai.
Based on Eastern Feng Shui geographical principle, Yin-Yang theory and the Five Elements, the Citadel faces the South. Architects chose mount Ngu Binh as the front screen, Hen islet, Da Vien islet to form Left Blue Dragon and Right White Tiger.
In its first construction phase in summer 1805, Nguyen dynasty called for about 30.000 locals and soldiers from localities in Central region to block river, dig trench and initially build a citadel from soil. The construction process took place in several years. Till 1818, the total number of soldier and employee joined in constructing is 80.000. They had the front side (the South) and the west side of the Citadel tiled. The East and North sides were tiled in 1822. After that, Emperor Minh Mang had works constructed on top of the outer wall in 1931 and 1832.
Hue Citadel looks like a square except for the front side which curved along the Perfume river.
The circumference of the brick wall is 10,571. The Citadel is 21,50m thick on average. The outer wall circle is 6,60m high while the inner one is 2,10m high. The total area of the Citadel is 520ha (5,20 km2)
There are 10 gates built around the Citadel in 1809. Two-storey watchtower were later constructed on top of these gates in 1824, 1829, 1831. In addition to the 10 main gates, there is a subsidiary gate without watchtower built to connect with Tran Binh fortress. Also, there are 2 irrigation works constructed on two ends of the Ngu ha river to enable its flow in trench system, dug river (known as Ho Thanh ha) and the perfume river.
Gates are named in accordance with the direction from Citadel center, namely Chanh dong gate, Dong Nam gate, Chanh Nam gate, Tay Nam gate, Chanh Bac gate, Dong Bac gate, etc. However, locals usually call them by familiar names such as Dong Ba gate, Thuong Tu gate, Nha Do gate, Huu gate, Hau gate, Ke Trai gate, etc.
These walls are topped by fortresses, watchtowers, etc for guarding and defending purpose. At the middle of the Citadel stands the giant solemn flag tower.
There are 2 waterways, namely trench and dug river (known as Ho Thanh ha) constructed outside of the solid wall circle. Besides, a fortification was established on the land lied between these two waterways. These three structures run along and support the Citadel. To facilitate transportation in the Citadel as well as adjacent areas, many bridges were constructed to connect the 2 waterways, especially in front of Citadel entrance such as Thanh Long bridge, Bach Ho bridge, Gia Hoi bridge, Dong Ba bridge, etc.
Hue Citadel is constructed on the basis of combining eastern architecture, military defense technique of Vauban (France) and terrain condition. This combination created a unique military art architecture as well as a national cultural heritage.
Being constructed for nearly 2 centuries and destroyed during wartime, Hue Citadel still keep its original look. The Citadel was recognized as national historical - cultural relics on May 12th, 1998 and considered as one of the most important art architectures in Hue Monuments Complex, world heritage.