In 1802 Nguyễn Ánh took the throne as King Gia Long (1802-1819), a name chosen symbolically to represent the union of the northern (Thăng Long) and southern (Gia Định) capitals. In the same year the royal seat of government was transferred to Huế. Construction of the new royal citadel on the north bank of the Hương (Perfume) River commenced in 1805 and continued until 1832. Substantially larger than any of its predecessors, the Huế Citadel was designed according to the doctrines of ancient geomancy and to principles of fortification developed by French military engineer Vauban (1633-1707). Surrounded by an outer moat and accessed via 10 entrance gates, the exterior enclosure (Kinh thành) comprised a 6.6-metre high perimeter wall with a base in excess of 21 metres thick. Covering a total area of 520 hectares, this exterior enclosure housed the Nguyễn government ministries, various offices of state and, at its centre, the 36-hectare Royal Enclosure (Hoàng thành). Protected by an inner moat and enclosed by a 4 metre-high stuccoed brick wall, the Royal Enclosure in its turn incorporated the various palaces and offices of the royal court, including ceremonial and reception rooms. At the heart of the Royal Enclosure was the Forbidden Purple City (Tử Cấm thành), which housed the private residence of the king and his family, royal concubines, eunuchs and ladies-in-waiting. The Forbidden Purple City had four gates, the most imposing being the South Gate (Ngọ môn), which was traditionally reserved for the king and is considered a masterpiece of Nguyễn architecture.

Similar treatment was meted out by the Nguyễn kings on the fast developing southern capital of Gia Định (Sài Gòn). Just 12 years before taking the throne as King Gia Long, Nguyễn Ánh had constructed a large fortification at Gia Định known as the Bát Quái Citadel, so called because of its resemblance to the Chinese 'Eight Trigrams' symbol. However, Minh Mạng considered this citadel to be too large for a provincial centre and in 1835 he ordered it to be demolished and replaced by a new smaller one built on the north eastern portion of the same site. This smaller fortification was in turn razed to the ground by the French after they had captured Sài Gòn in 1859, to ensure that it could not be used against them by the Vietnamese.

Abandoned after the abolition of the monarchy in 1946, the royal palaces within the Forbidden Purple City of Huế fell into a state of serious disrepair long before the Tết Offensive of 1968, when the entire complex became the backdrop for one of the bitterest battles of the American War. By 1975 only 20 out of the original 148 palaces and administrative buildings were left standing. Severe weather conditions have also caused erosion to the site. Since 1996 significant progress has been made in restoring the surviving monuments.

Other important architectural relics of the Nguyễn dynasty include the Nam Giao Esplanade, the Royal Arena, the Temple of Literature and numerous royal tombs, notably those of Tự Đức, Minh Mạng and Khải Định which incorporate several imposing stone statues and stelae. Also of architectural note were the large defensive citadels constructed by the Nguyễn kings at Sơn Tây in 1822 and at Quảng Trị in 1872, though little now remains of either structure.

Notwithstanding occasional campaigns to limit the influence of Buddhism, the Nguyễn kings continued to patronise the establishment of Buddhist pagodas. Construction in the north remained somewhat limited throughout the Nguyễn era, but Tự Đức (1847-1883) founded numerous pagodas in Huế and throughout the 19th century a large number of pagodas were constructed in the south, notably Phong Sơn, Giác Lâm and Giác Viên (Sài Gòn) and Linh Sơn (An Giang Province).

Domestic architecture also experienced noteworthy development in Huế during this period with the construction of elegant and richly-carved wooden residences surrounded by large ornate gardens for the families of royal relatives, courtiers and other wealthy individuals. An elegant fusion of royal and folk architecture, the typical Huế nhà vườn or 'garden house' ranges in style from the simple nhà rội, constructed around one central column, to the nhà rường, constructed with the frame resting on two or more columns, and the nhà thượng rường, a combination of these two types. Subsidiary buildings such as kitchens, servants' lodgings, warehouses or stables may frequently be found within 'garden house' enclosures, along with water wells for daily use and ornamental features such as rockeries or aquaria.