Appropriate preposition before a city

Question

Normally I have used the preposition à before a city, as in je vais à Paris.  You don't use au Paris.  In this exercise, we have a city with a plural name, namely Les Sables d'Olonne.  Apparently, one must use aux Sables d'Olonne in stead of à Les Sables d'Olonne.  So, is this a general rule: à + name of a singular city and aux + name of a plural city name?

Accepted Answer

If the article is part of the place/city name, the normal rules of contraction or otherwise with a preposition apply. 
See attached lesson - Le Havre and La Rochelle as examples.
 https://progress.lawlessfrench.com/my-languages/french/view/3119

Answer

Thanks Maarten,  I now remember this lesson and, in fact, had it bookmarked.   I have put grammar on a back burner to favor listening comprehension.   I guess I will have to revisit certain grammar points from time to time.  BTW, sorry for the misspelling of rote.  Your answers are greatly appreciated.

Answer

Hi Frank, this isn't so much a grammar point but just that à + les = aux. You need the article les in this case because it forms part of the name of the city.