This is a curious WHS, and I wonder how many Westerners have visited it before its designation in 2010. The 2009 Lonely Planet Vietnam describes it as a "military base" and "closed to the public". News updates on the web that I had read up on before said that it had opened for a while last year, and then closed again for renovations. So I had no idea of what to expect when I set out to find it.
The entrance to the citadel, the main part of the WHS, turned out to be pretty inconspicuous: you surely would not go in if you weren't looking for it. It is located around the corner from the Red Flag Tower, and most signs pointing to it are in Vietnamese. While the nearby Ho Chi Minh Mausoleum sees thousands of domestic and foreign visitors a day, here I encountered only about a dozen Vietnamese. The entrance is free and a visit (at least my one did) lasts about an hour.
What you'll find here is a number of gates and buildings, all in yellow and most of them originating from the 19th century. The site is hard to date for our "Built in the xth century"-connection, as Thang Long's status as capital lasts from as early as 1010. Most structures here have been rebuilt however during the course of time.
I did not visit the Archeological Site, which is located across the street from the citadel. It has very limited opening hours (a couple of hours a day on weekends), and was closed when I arrived there.
Seeing the state of the remains, it is doubtful if this is a worthy WHS. Hanoi got this status last year as a present for its 1000th birthday, after a clever nomination from the Vietnamese. It compares itself to Nara (Japan) and Xi'an (China), but its remains are much less imposing. The lovely capital of Vietnam surely merits international attention though, especially for its Old Quarter (which is about 2 km away from the citadel).