Victor Hugo is a colossus of Universal Literature and Notre Dame de Paris (The Hunchback of Notre Dame) is one of his most important works. Claude Frollo, Esmeralda and Quasimodo are the most remembered characters of this romantic tragedy, but the true main character of the novel is the Notre Dame Cathedral; hence the original name of the book.

It was published in 1831, but the story takes place in the fifteenth century and Victor Hugo took the opportunity to denounce the demolition of Gothic buildings that he considered should be preserved as cultural heritage. Although he had already written several articles about it, it was the story of this book, endowed with memorable characters and majestic descriptions of the construction and evolution of both the cathedral and the city of Paris, which helped to preserve and restore several of those buildings.
In addition, Notre Dame is the scene of the most transcendental moments of the plot and also from there many others are attested. Like the relationship of mimesis, the identification between Quasimodo and Notre Dame, as the hunchback "...had taken its form, as the snail acquires that of its shell". He was the soul of the Cathedral. A tormented soul because he had grown practically enclosed doubly: in the church and in his deformed body and "... the soul atrophies in a deformed body". He was bad because he was wild and he was wild because he was ugly.

Quasimodo, bell boy of Notre Dame is red-haired and deformed; not only he's hunchbacked, he's also lame, one-eyed and deaf. In contrast to him, is Esmeralda, the beautiful gypsy girl with black eyes and hair. Both are diametrically opposed physically, but both are compassionate and help each other in times of need, in front of the indolence of the people. They are two marginalized, he by deformed, she by gypsy, who only crave happiness, she with Captain Phoebus and he with the gypsy herself, but whose circumstances (society, power, race, origin, appearance) condemn them to never achieve their yearnings.
Among many remarkable things that the novel has, I will talk about three that caught my attention. The first is that there is a passage in the novel, which suggests the immaculate conception of Esmeralda in her mother's womb:

“Unable to have a lover, she put her desires into having a child, and since she was still a good Christian, she asked God fervently. God felt sorry for her and gave her a child.”

Is it the gypsy of good heart, condemned to die on the gallows, a kind of avatar of the biblical Jesus? Saving the differences, of course, any immaculate conception will evoke, immediately, the Christian myth.
The second to be highlighted is the purity of the love that Quasimodo professes to Esmeralda. The hunchback felt condemned to see happiness in others, it was bitter to see couples because he was convinced that he would never live this happiness. By helping Esmeralda out of gratitude (she had helped him before), he has no illusions. He knows he's ugly and doesn't long her to love him. Moreover, he understands the discomfort of the young woman before his ugliness and tries not to scare her:

“What can you care that I am there when you have your eyes closed? Now I'm leaving. I am already behind the wall; you can open your eyes.”

Seeing her suffer for Phoebus, he offers to go and look for him, but seeing that he's with another woman, he hides the truth from the gypsy because he "preferred to be mistreated by her rather than to afflict her". With sad resignation he sings at night:

“Don't look at the figure,
look, girl, to the heart[…]
beauty loves only beauty...”

That love makes him confess silently through a gesture: one morning he leaves two glasses of flowers next to Esmeralda. One is made out of glass, but it is cracked and therefore the flowers, having lost the water, have withered. The other is a simple ordinary clay vase with the flowers intact and its interior filled with water. That image is poetry.
However, the gypsy, and here's the third thing I want to emphasize, chose the wilted flowers (consciously?) And kept them in her chest. Did we think she would fall in love with the hunchback? Esmeralda is a beautiful girl of 16 years old, who loves in her adolescent feeling the gallant Captain Phoebus de Chateaupers and although he doesn't correspond to her, she doesn't see her salvation in the beauty of the feeling that Quasimodo professes to her.

Víctor Hugo included in this novel a wide spectrum of dualities and antagonistic characteristics: beauty and ugliness, mainly in Quasimodo and Esmeralda; nobility and marginality in the King and the Court of Miracles with their false cripples; carnal passion and spirituality in the person of Claude Frollo, the priest who falls in love with the gypsy; likewise, in the person of the priest, they confront the Catholic faith and alchemical mysticism, heresy condemned in the Middle Ages; the double morality of a society that proclaims itself Christian, but criticizes and condemns others because of their race, origin, clothing or appearance; charity and indolence; leaving room also for social and political criticism (the judge who condemns Quasimodo is literally deaf, but this doesn't prevent him from exercising his office and distributing "justice"), which added to the romance, tragedy, descriptions of the scenarios, gives the novel an impressive fullness that would crystallize the same author three decades later in his monumental work Les Miserables.
A Classic among Classics, without a doubt, The Hunchback of Notre Dame is a jewel of universal literature and therefore I recommend it to anyone who wants to immerse themselves in its reading.