Hue was the capital of Vietnam from 1802 to 1945, the seat of all thirteen Nguyen emperors. The citadel they built on the north bank of the Perfume River is a scaled-down copy of Beijing's Forbidden City — a walled capital with three concentric enclosures: the outer citadel (shops, workshops, officials), the Imperial City (state buildings, Thai Hoa throne hall), and the innermost Forbidden Purple City where the emperor and his immediate household lived.
Most of the inner complex was destroyed in 1968 during the Tet Offensive — the citadel was occupied by North Vietnamese forces for 25 days and retaken only after heavy US and South Vietnamese artillery. Of 160 original buildings, about 30 survived. Since UNESCO listing in 1993, reconstruction has been steady: the Royal Theatre, the Temple of Generations (shrine to the thirteen emperors), several ceremonial halls. It's a working conservation site — something is under scaffolding most months.
What still stands is worth the trip. The Ngo Mon Gate (main entrance with its yellow-tiled five-phoenix pavilion), the Thai Hoa Palace (throne room with 80 carved red-and-gold columns), the Nine Dynastic Urns cast in bronze in 1835, the Royal Theatre — each is preserved in working condition. The tombs outside the city, particularly Minh Mang and Khai Dinh, rival the citadel for architectural interest. Plan on a full day.