Walk the spiral earth ramparts of the country's oldest citadel
Third century BC, built in rings
Low grassy earthen banks several metres high running in long curves through farmland and villages, enclosing a temple to the king who built them and a shrine to his daughter.
Why it's unique
Co Loa was built in the third century BC as the capital of the Au Lac kingdom, laid out as concentric spiral ramparts rather than the square walls used almost everywhere else in the region. Nine rings were built; three survive.
It is the oldest fortress in the country and the most ancient urban site in it, and the spiral plan is unusual anywhere in the world. What survives is earth rather than stone, which is why it reads as landscape until you understand what you are standing on.
Before you go
The single most common mistake is to visit the temple compound, see a temple, and leave. The ramparts are the monument. Walk along the top of the outer bank for a few hundred metres — it is the only vantage from which the concentric plan becomes legible rather than looking like farm bunding.
There is almost no English interpretation on site, so read the outline of the An Duong Vuong story before you go or the shrines will not mean much. Roughly 17 km north of the centre; a half day with travel, and best combined with nothing else.