La Chaux-de-Fonds is located in the far west of Switzerland, a bit off the beaten track, but I combined it with Besancon (Vauban citadel) and Arc-et-Senans, as there are around 3 daily trains from Besancon. I spent half a day in La Chaux-de-Fonds (plus around 40 minutes in Le Locle, only 10 minutes away by train, but my visit was cut short by a huge thunderstorm), and I felt that this was enough to get a feel for the city. I joined a tourist train for a ride (especially useful for the residential districts located on a quite steep slope) and so could see the rectangular street layout (also well visible from the Espacité lookout tower) and some of the unique buildings of this town. I also visited the Watchmaking Museum, which was quite interesting, but only focused on watches/clocks in general, not on the city itself. Le Locle is much smaller than La Chaux-de-Fonds, but quite similar in its structure and buildings (it also has its own watchmaking museum). The WH designation is probably justified for the unique city plans and the focus on a very specific type of precision industry, away from the usual ironworks/steel mills/mining sites. The two cities are nice places to visit if you are in the area, but going out of the way for them is only something for WH collectors.