I visited St. Denis in 2016, and it has been in a squalid suburb for a long time since the 19th century. We took the metro and there were a few tourists on the train but as you would expect, most people were of north African origin (former French colonies Algeria, Tunisia, Morocco).
It’s one of if not the first major church to use stained glass which was an innovation by the abbe Suger.
It’s also the necropolis for many the French kings, something of the French version of Westminster abbey. Many of the tombs and statutes were damaged during the French revolution.
I’ve been there, too. I believe it was the first of the gothic style basilicas, with the flying buttresses.