Here's what you should know about this novel's style: it has a huge scope, and it goes from major historical events to the little mundane happenings on the streets of Paris. For Hugo, everything is connected. Just look at how he explains Quasimodo's profession:
He had been for several years bell ringer of the cathedral of Notre-Dame, thanks to his foster father, Claude Frollo, who had become Archdeacon of Josas, thanks to his lord, Louis de Beaumont, who had been appointed Bishop of Paris in 1472, thanks to his patron Olivier le Daim, barber to Louis XI, by the Grace of God, et cetera, et cetera. So Quasimodo became bell ringer of Notre-Dame. (IV.III.1)
See how the language zooms farther and farther out, until we're talking about the King of France and then the Grace of God—only to return back to little ol' Quasimodo in his bell tower? Hugo loves doing this kind of thing: he loves situating his characters in real, complex historical situations. He even does it when he describes Louis XI's bed in the Bastille, but this time he jumps forward in history:
This bed, famed for having witnessed the sleep or sleeplessness of Louis XI, was still to be seen two hundred years ago in the house of a councilor of State where Madame Pilou saw it. She was immortalized in the Cyrus under the name of Arricidie and La Morale vivante. (X.V.7)
Wait, what? Why is Hugo referencing some book from the 17th-century? (Also, thank goodness for footnotes.) Part of the reason might be that he's showing the reader that you can't make this kind of stuff up—that Louis XI really did sleep in a plain old bed. Or, as in the passage above, he's showing you how history is all about how things are related.
After all, the way things are related is a big theme in the novel: remember how in the preface the author comes across some medieval graffiti that piques his curiosity? It's the old graffiti that gives him the spark to write the novel. The same sort of thing goes on throughout The Hunchback of Notre-Dame.Things don't just exist in a vacuum; everything has some sort of story behind it.