The Citadel was built by French officer Olivier de Puymanel for Nguyễn Ánh in 1793 with an area of 36,000 square meters according to the Vauban military architecture which was popular in Western Europe in 17th - 18th centuries.

The outer face was vertically constructed while the inner was a little bit sloping by two terraces forming a favorable pavement.

Inside the corners, there were large fields which were convenient for military residents.

On the top of the corners there stood fortresses 2m high with canons above.

There was also a royal palace, private residents of feudal mandarins, warehouses and jails.