Unique architecture of Son Tay Ancient Citadel | Hanoi is beautiful and not so beautiful Built in 1822 under the reign of King Minh Mang 3, Son Tay Ancient Citadel has an area of ​​up to 12 hectares. Of the 20 citadels built during the Nguyen Dynasty, only Son Tay Citadel was built entirely of laterite bricks - a material that meets the durability requirements of a defensive structure, and is very readily available in the country. Doai