The construction of the Citadel started in 1805 during the reign of Emperor Gia Long of the Nguyen Dynasty. The successive kings of the dynasty have ordered to build ramparts, palaces and other structure for royals. The exterior citadel has 24 bastions on the top of the wall for defensive purpose.

At the center of the Citadel, locates the Imperial City which was the seat of the highest offices of Vietnam's Feudalism. The Imperial City's compound is connected with the outer citadel by four entrance gates, among which the Noon Gate is only used for the king. The area of Imperial city is divided into sectors, packed with more than 100 beautiful buildings and structures.

The Forbidden Purple City is also located in the Imperial City. This area is uniquely reserved for Emperors and his royal family. More than 50 architectural buildings were built in different shapes and different purposes, including the Emperor's Private Palace, palace for conducting political affairs, Queen's residence, royal theatre, Emperor's library, kitchen, etc.