The citadel, a UNESCO world heritage site in Hue Town of Thua Thien-Hue Province, was home to the Nguyen Dynasty (1802-1945), Vietnam's last royal family.

The first gate, situated to the right of Luong Y Bridge, is 80 cm wide and 100 cm high. The brick floor beneath the bridge arch was found to be intact.

It was discovered following the demolition of homes built on the Thuong Thanh relic in Thuan Loc Ward of Hue that saw residents relocated earlier this month.

The second gate, situated to the left of Luong Y Bridge, sits behind a house built decades ago.

Researcher Nguyen Xuan Hoa, former director of Thua Thien-Hue's Department of Culture and Sports, said historic Nguyen Dynasty records do not mention the two gates.

Within the entire imperial citadel, two epitaphs located by Khanh Ninh Bridge and Kho Bridge bear information on the formation of Ngu Ha River to the east of Thuy Quan, where both gates are situated. But there is no information about the gates themselves, which are big enough for one person per entry, he said.

Hoa added that in the past, Thuy Quan served as an important defensive post for the Nguyen Dynasty. Boats wanting to enter the Hue Imperial Citadel on Ngu Ha River had to pass through Thuy Quan, guarded by 13 gun or cannon placements.

"Perhaps these two small gates served Nguyen Dynasty guards at Thuy Quan, monitoring boats passing from Dong Ba River to Ngu Ha River. Another theory holds they may have been an escape route for the royals," said Hoa.

Dr Tran Dinh Hang, director of the National Institute of Culture and Arts of Vietnam in Hue, said that the newly discovered gates would help researchers learn more about the architecture of the citadel.

"The two gates are very mysterious since there are no record of their existence. They were most likely used as an escape route," Hang said.

One of the gates is blocked by bricks. Photo by VnExpress/Vo Thanh.

The Hue Imperial Citadel was built under King Gia Long (1802-1820), the first Nguyen Dynasty Emperor, and restored under King Minh Mang (1820-1839), the second Nguyen Emperor.

The citadel sits on area of 520 ha. Its perimeter covers nearly 10 km while its surrounding walls are 6.6 m high, and 21 m wide. The citadel has 10 main gates facing different directions.

Since 1945, thousands of households had settled in areas within its walls.

Thua Thien-Hue provincial officials announced the relocation plan in 2018. In March, 523 households near Thuong Thanh relic dismantled their homes themselves and relocated to Huong So. The slum, which has an estimated population of 15,000, stands in what is known as Hue Relic Area 1.

More will be relocated this year from other relics across the citadel.