Sơn Tây Citadel was built by King Minh Mạng in 1822 to defend the western gateway to the city of Thăng Long (Hà Nội). Constructed mainly from laterite, the four-sided citadel featured a 1,303 metre perimeter with a 4.4 metre high wall, surrounded by a 20 metre wide moat. The foundations of various buildings may still be seen within the complex. The structure was almost completely destroyed by the French in 1883, leaving only the north and south gates standing. While the north gate has been restored unsympathetically using concrete, the south gate remains almost intact.