The ancient, spiral-shaped citadel dates back to the third century BC, with a few of the ancient ramparts remaining but hard to spot among the more recent construction.
There's a reflecting pond with a statue of King An Duong Vuong shooting a bow and arrow. The weapon had magical powers and he was supposed to use it to fight off Chinese invaders, but his enemy got hold of it and used it to defeat him, ushering in nearly a century of Chinese rule.
There are three sites to visit here: a small pagoda dedicated to the king's daughter (who figured prominently in the tale of woe behind the temple's history), the Am Mi Chau pagoda, which now houses a museum displaying archaeological finds from the area dating back as much as 5,000 years (though you'll find more of the same and better at the History Museum in Hanoi), and also the temple dedicated to the king himself, which has a good display of Buddhist statuary dating back several hundred years.
There's a lot of lore and history here, more so than the other sites listed in this section, so try to hire a guide in Hanoi to give you the rundown.
Admission gives access to all the sites, so keep your ticket handy.
How to get there
The Citadel can be reached by leaving Hanoi to the northeast. Take the Chuong Duong bridge over the Hong River out of town to where the road joins up with Highway 1. You'll cross another river 11km later via an old, narrow steel bridge. Take the immediate left, and follow that road as it meets Highway 3, keep straight, and after about 4km there's a sign on the left marking the turn for the site on the right. Turn off for the Citadel. It's another kilometre further down that road and well marked.