Much of the country was devastated in the American War, but a few ancient towns survived. Hoi An is the most popular of those. Almost all buildings in the old center have stood for centuries, and even newer neighborhoods try to be sensitive of the past.
I have occasionally complained about towns that sold their soul to tourism, like Aguas Calientes or Sapa, but Hoi An manages to absorb the inevitable glut of restaurants and souvenir shops with grace. And the tourists all seem to confine themselves to the few main streets and don’t venture into the many small alleys. It’s also somewhat upmarket: there are few budget options and few backpackers, and few pizza places or bars that serve alcohol in buckets. A very pleasant place to stay.