The Citadel in Hue, Vietnam. Inside the Citadel is the Imperial City, with a perimeter of almost 2.5 kilometers. Inside the Imperial City was the imperial enclosure called the Purple Forbidden City in Vietnamese, a term similar to the Forbidden City in Beijing. (Source: Wikipedia). In 1968, the Citadel became a battleground where the North Vietnamese Army fought the Army of the Republic of Vietnam and the American Army resulting in very high casualty rates on both sides, including among the civilian population.