The Imperial Citadel of Thang Long is the only UNESCO World Heritage Site within Hanoi; the only other nearby UNESCO site is Halong Bay, which I visited my first weekend in Vietnam. I wandered upon it late in my stay in the northern Vietnamese city — long after I was already exhausted from life in the Old Quarter. The highlight of this historic site at the time was that it wasn’t crowded and there was no one inside trying to sell me things I didn’t want.
I wasn’t specifically searching for the Imperial Citadel, but I had seen it mentioned when I browsed UNESCO sites, so I was subconsciously keeping an eye out for it as I attempted to escape the crowds in Hanoi. I came across this piece of Vietnamese history after a morning of wandering around the Ho Chi Minh mausoleum and presidential palace.
The citadel is next to the military history museum, but I didn’t bother visiting that one — I was tired and wasn’t really interested. It’s also just across from a park with a statue of Vladimir Lenin — I’m sure he’d appreciate the teenager lounging at his feet and the couple dancing nearby.
The Imperial Citadel of Thang Long was first built as a palace and other structures by the Lý Dynasty in 1010 and expanded by the Trần, Lê and Nguyễn dynasties. A more modern citadel still remains at the site, including the flag tower that was built in 1812 under King Gia Long, but most of the original structures were destroyed over the centuries. 
The remains of the original imperial city were discovered in 2008 when Ba Đình Hall, the old parliament building, was torn down to make way for a new one.
Today, there isn’t much to see around the Imperial Citadel of Thang Long. There are active archaeological dig sites and some protected ruins, but not much to actually see — these are not like the ruins of Rome. While I was there, I saw a group of students who appeared to be taking graduation photos (I guess they were taking the photos a semester early as this was early January).
As I didn’t know much of the history of Thang Long at the time, I didn’t find the citadel as awe inspiring as other palaces I’ve visited — it certainly isn’t as interesting as the palaces in Bangkok or Phnom Penh, or even the Forbidden City when it isn’t shrouded in scaffolding. Unfortunately, historic sites in Vietnam haven’t yet developed educational self-guided tours (aside from the propaganda at the war museums), but I hope more of this will be developed for Thang Long as more of the palace is unearthed.
Have you ever visited an historic site only to be disappointed until much later when you learn more about it?
Thought Olympia in Greece was like that too. Didn’t help that ALL the signs on site were “under repair” when we went so you were just walking around a park with various rocks scattered here and there. Read up and appreciated Olympia more after.