From the World Heritage inscription:
Established as the capital of unified Viet Nam in 1802, Hué was not only the political but also the cultural and religious center under the Nguyen dynasty until 1945. The Perfume River winds its way through the Capital City, the Imperial City, the Forbidden Purple City and the Inner City, giving this unique feudal capital a setting of great natural beauty.
Hue is analogous to other Asian cities such as Kyoto, Gyeongju and Nanjing in that it is a former imperial city. Unlike the other cities, the history doesn’t date back nearly as far. The current structures only date back to the early 19th century.
There actual city of Hue is pretty large; almost one million people. The history part of the city consists of the walled outer city and the walled inner city, which contains the royal residences. Most of the royal buildings are undergoing renovation/reconstruction and there currently isn’t much to see.
In addition to the walled city, the Thien Mu Pagoda (shown above) is also part of the World Heritage site and is located on the river several kilometers out of the city. You can easily get there via bicycle rickshaw.