What I found was that the 10 municipalities which have been hardest hit in proportion to population (175 to 300+ hospital cases per 100,000 inhabitants) have something in common. They are in the Catholic areas of the country (except Oudewater, which has a long history of tolerance towards Catholics), and several of them are renowned for their Carnival festivities. Moreover, they're not so far away from Tilburg, where the first confirmed case of the virus in the Netherlands was reported on February 27th. The Carnival week-end was February 28th/29th. Confirmed cases of the virus multiplied by 8 or so between March 4th and 9th, by which time a third of those cases were in the Noord-Brabant province, which includes Tilburg.
In contrast, in the highly populated areas, the cases per population are far lower. I looked at the statistics for the 15 most densely populated municipalities in the country, including Amsterdam. They ranged from 20.3 per 100,000 in Krimpen aan den Ijssel (coincidentally, where I lived when I was there) to 43.2 in neighbouring Capelle aan den Ijssel. Odd! Two places on opposite sides of a river, connected by a short bridge, with such different infection rates? And in both cases, a lot of their working residents do their work in Rotterdam? Mmmm... Capelle, 40 years ago at least, was mostly blocks of high-rise flats, each surrounded by greenery. Krimpen, while closely packed, was low-rise; mainly conventional two-story houses.
What this suggests to me is that the virus spreads most rapidly when there are a lot of people in close proximity, as at Carnival and in high-rise blocks. It isn't how far you keep away from the next person that matters; it's how far you keep away from crowds. And that may provide a reason why the Austrians have done so well, relatively, in this epidemic. When they had a major problem with patients who had been to Ischgl, they quarantined the whole town. The Icelanders also took this approach, banning large public assemblies, but only putting individuals into lockdown in one small area.
Am I on right lines, or am I way off base?