We were kind of tempted to put Notre-Dame in the "Characters" section simply because it plays such a huge role in this novel. You should definitely read our "What's Up with the Title?" section if you want to get a sense of just how central to the novel the cathedral itself is. Think of it as the hub and all of the characters as spokes revolving around it.
None of the characters' lives would intertwine at all if it were not for the way the cathedral serves as a center for all of the events, the place where all the characters meet and intersect. For that matter, the story wouldn't exist at all if someone hadn't bothered to carve something on one of the cathedral's walls—a carving that lasted for hundreds of years.
Okay, so the cathedral is central to the novel. But what does this mean? We're going to zero in two major aspects of Notre-Dame: 1) the fact that it seems to be alive and 2) the fact that it's permanent, kind of.
In the chapter "Notre-Dame" (Book III.I), the narrator calls the cathedral "a transitional edifice" (III.I.17) and "the work of ages" (III.I.21). Basically, he says, Notre-Dame is not in one single, stagnant style: it changes with every era. In this sense, it's kind of a symbol for Time itself, and for the changes brought on by time. Old things disappear and are replaced by new things—just think of Frollo's wall carving, which lasts for a few centuries only to be covered up.
So Notre-Dame, even though it's made from seemingly unchanging stone, is actually pretty dynamic. Hugo loves to play with the idea that the cathedral is "alive," and sometimes this almost seems to be literally true:
The flickering of the flame made them seem as if the sculptures were alive. Gorgons seemed to be laughing, gargoyles howling, salamanders puffing fire, and griffins sneezing in the smoke. (X.IV.34)
You think Notre-Dame is going to let the characters have all the fun? Think again. Our Lady is dressed to the nines and ready to go hard. But seriously, by making the cathedral take on a life of its own, Hugo emphasizes that it has a role to play, just like everyone else.
And it's not just that the cathedral is a big mix of dynamic architectural styles. It's also a place where thousands of people's lives have intersected and influenced each other. Think about all the people have passed through Notre-Dame's doors. That's a lot of people, and each one brings something unique to it.
In a way, the cathedral is alive with the thoughts, emotions, and desires of all of those people—and people from totally different time periods can actually influence each other, just as that anonymous graffiti-carver influenced Hugo to write an entire novel about the cathedral, centuries later.
And It's Here to Stay
Okay, so what about the second aspect of Notre-Dame: its permanence? We talk about this idea a lot in our "What's Up with the Epigraph?" and "What's Up with the Ending?" sections, but the gist of it is that a cathedral is much more permanent than, say, a human life.
In order for a person's history to be told, something has to remain long after that person is gone. During the Middle Ages, cathedrals like Notre-Dame (which has managed to stay around for 850 years with no signs of going anywhere) were the most permanent things out there, and they provided people with the best means for passing on something of themselves to future generations.
Now, the idea of the cathedral as something permanent may seem a bit at odds with the idea of it as something mutable and dynamic. After all, can something be both permanent and always changing? It's a tricky idea, for sure, but we think that's what Hugo is getting at. Certainly not everything about Notre-Dame is permanent—its architecture changes, its graffiti gets whitewashed over, and its statues and windows get destroyed and replaced—but Notre-Dame as a structure and as an idea has remained.
Think of the cathedral as a link from the past to the present. Everything around it might change—Paris will change; old generations will die and new ones will be born—but the cathedral is always there. It stands in contrast to all the disappearing things around it. We see more or less the same cathedral people saw in 1482.
Think about how, in the Author's Preface, Hugo is able to glimpse a bit of the cathedral's long-gone past in the word ANÁΓKH. Now contrast this with the image of Quasimodo's skeleton falling to dust at the very end of the novel. Hugo is using the ideas of transience and permanence to talk about history. Some things stay, and some things go. And hey, what better symbol is there of permanence than a huge block of stone?