His major restoration projects included Notre-Dame de Paris, the Basilica of Saint Denis, Mont Saint-Michel, Sainte-Chapelle, the medieval walls of the city of Carcassonne, and Roquetaillade castle in the Bordeaux region.

His writings on decoration and on the relationship between form and function in architecture had a fundamental influence on a whole new generation of architects, including all the major Art Nouveau artists: Antoni Gaudí, Victor Horta, Hector Guimard, Henry van de Velde, Henri Sauvage and the École de Nancy, Paul Hankar, Otto Wagner, Eugène Grasset, Émile Gallé, and Hendrik Petrus Berlage.

His uncle Étienne-Jean Delécluze was a painter, a former student of Jacques-Louis David, an art critic and hosted a literary salon, which was attended by Stendhal and Sainte-Beuve.

Instead he decided to get practical experience in the architectural offices of Jacques-Marie Huvé and Achille Leclère, while devoting much of his time to drawing medieval churches and monuments around Paris.

"[5] He was a talented and meticulous artist; he travelled around France to visit monuments, cathedrals, and other medieval architecture, made detailed drawings and watercolours.

In 1838, he presented several of his drawings at the Paris Salon, and began making a travel book, Picturesque and romantic images of the old France, for which, between 1838 and 1844, he made nearly three hundred engravings.

[9] In October 1838, with the recommendation of Achille Leclère, the architect with whom he had trained, he was named deputy inspector of the enlargement of the Hôtel Soubise, the new home of the French National Archives.

He needed a magnificent audacity to take charge of such a desperate enterprise; it's certain that he arrived just in time, and if we had waited only ten years the church would have been a pile of stones.

In February 1843, King Louis Philippe sent him to the Château of Amboise, to restore the stained glass windows in the chapel holding the tomb of Leonardo da Vinci.

They proposed two major changes to the interior: rebuilding two of the bays to their original medieval height of four storeys, and removing the marble neoclassical structures and decoration which had been added to the choir during the reign of Louis XIV.

When not engaged in Paris, Viollet-le-Duc continued his long tours into the French provinces, inspecting and checking the progress of more than twenty different restoration projects that were under his control, including seven in Burgundy alone.

In 1856, using examples from other medieval churches and debris from Notre-Dame as his model, his workshop produced dragons, chimeras, grotesques, and gargoyles, as well as an assortment of picturesque pinnacles and fleurons.

He also undertook an unusual project for Napoleon III; the design and construction of six railway coaches with neo-Gothic interior décor for the Emperor and his entourage.

When he wished to put up a monument to mark the Battle of Alesia, where Julius Caesar defeated the Gauls, a siege whose actual site was disputed by historians, he asked Viollet-le-Duc to locate the exact battlefield.

In September, the Emperor was captured at the Battle of Sedan, a new Republican government took power, and the Empress Eugénie fled into exile, as Germans marched as far as Paris and put it under siege.

"[24] Always the scholar, he wrote a detailed study of the effectiveness and deficiencies of the fortifications of Paris during the siege, which was to be used for the 1917 defense of Verdun and the construction of the Maginot line in 1938.

He escaped to Pierrefonds, where he had a small apartment before going in exile in Lausanne, where he engage in his passion for mountains, making detailed maps and a series of thirty-two drawings of the alpine scenery.

He received his only commission from the new government of the French Third Republic; Jules Simon, the new Minister of Culture and Public Instruction, asked him to design a plaque to be placed before Notre-Dame to honor the hostages killed by the Paris Commune in its final days.

Viollet-le-Duc designed all the work to the finest details, including the floor tiles, the gas lights in the salons, the ovens in the kitchen, and the electric bells for summoning servants.

He spent more and more time at La Vedette, the villa he constructed in Lausanne, a house on the model of a Savoyard chalet, but with a minimum of decoration, illustrating his new doctrine of form following function.

(2) The restoration had to involve not just the appearance of the monument, or the effect that it produced, but also its structure; it had to use the most efficient means to assure the long life of the building, including using more solid materials, used more wisely.

"[34] During the entire career of Viollet-le-Duc, he was engaged in a dispute with the doctrines of the École des Beaux-Arts, the leading architectural school of France, which he refused to attend as a student, and where he taught briefly as a professor, before being pressured to depart.

In 1846 he engaged in a fervent exchange in print with Quatremère de Quincy, the Perpetual Secretary of the French Academy, on the question, "Is it suitable, in the 19th century, to build churches in the Gothic style?"

The flying buttresses and contreforts alone support the entire structure, and always have an aspect of resistance, of force and stability which reassures the eye and the spirit; The vaults, built with materials that are easy to mount and to place at a great height, are combined in an easy disposition that places the totality of their weight on the piles; that the most simple means are always employed...and that all the parts of these constructions, independent of each other, even as they rely on each other, present an elasticity and a lightness needed in a building of such great dimensions.

Many art historians also consider that the British architectural writer John Ruskin and William Morris were ferocious opponents of Viollet le Duc's restorations.

His cardinal principle was to retain but not to restore the surviving remains of an ancient structure; and in this respect he departed emphatically from the tradition of Viollet-le-Duc and his successors in France and Italy, where exuberant restoration frequently obscured the evidence upon which it was based ...”[41] Throughout his career Viollet-le-Duc made notes and drawings, not only for the buildings he was working on but also on Romanesque, Gothic and Renaissance buildings that were to be soon demolished.

His study of medieval and Renaissance periods was not limited to architecture but extended also to such areas as furniture, clothing, musical instruments, armament, and geology.

Sir John Summerson wrote that "there have been two supremely eminent theorists in the history of European architecture – Leon Battista Alberti and Eugène Viollet-le-Duc.

Many of his designs emphasizing iron would later influence the Art Nouveau movement, most noticeably in the work of Hector Guimard, Victor Horta, Antoni Gaudí and Hendrik Petrus Berlage.

He was so influenced by the conflict that during his later years he described the idealized defense of France by the analogy of the military history of Le Roche-Pont, an imaginary castle, in his work Histoire d'une Forteresse (Annals of a Fortress, twice translated into English).