Eugène Viollet-le-Duc in the context of "Antoni Gaudí"

Modern architecture, also called modernist architecture, or the modern movement, is an architectural movement and style that was prominent in the 20th century, between the earlier Art Deco and later postmodern movements. Modern architecture was based upon new and innovative technologies of construction (particularly the use of glass, steel, and concrete); the principle of functionalism (i.e. that form should follow function); an embrace of minimalism; and a rejection of ornament.

According to Le Corbusier, the roots of the movement were to be found in the works of Eugène Viollet-le-Duc, while Mies van der Rohe was heavily inspired by Karl Friedrich Schinkel. The movement emerged in the first half of the 20th century and became dominant after World War II until the 1980s, when it was gradually replaced as the principal style for institutional and corporate buildings by postmodern architecture.

Architectural theory is the act of thinking, discussing, and writing about architecture. Architectural theory is taught in all architecture schools and is practiced by the world's leading architects. Some forms that architecture theory takes are the lecture or dialogue, the treatise or book, and the paper project or competition entry. Architectural theory is often didactic, and theorists tend to stay close to or work from within schools. It has existed in some form since antiquity, and as publishing became more common, architectural theory gained an increased richness. Books, magazines, and journals published an unprecedented number of works by architects and critics in the 20th century. As a result, styles and movements formed and dissolved much more quickly than the relatively enduring modes in earlier history. It is to be expected that the use of the internet will further the discourse on architecture in the 21st century.

A fortified house or fortified mansion, from French maison forte [fr], is a type of building which developed in Europe during the Middle Ages, generally with significant fortifications added. During the earlier Roman period it was common for wealthy landowners to construct unfortified villas on their lands. After the fall of Rome, increased social instability and military conflict necessitated more austere, defensible types of structures. A castle is a type of particularly well-fortified residence. In English-language academic works of castellology, the French term maison-forte is also used, more seldom unhyphenated (maison forte).

A cavalier is a fortification which is built within a larger fortification, and which is higher than the rest of the work. It usually consists of a raised platform within a fort or bastion, so as to be able to fire over the main parapet without interfering with the fire of the latter. Through the use of cavaliers, a greater volume of fire can be obtained, but its greater height also makes it an easier target for a besieger's guns.

Victor Pierre Horta (French: [viktɔʁ ɔʁta]; Victor, Baron Horta after 1932; 6 January 1861 – 8 September 1947) was a Belgian architect and designer, and one of the founders of the Art Nouveau movement. He was a fervent admirer of the French architectural theorist Eugène Viollet-le-Duc and his Hôtel Tassel in Brussels (1892–93), often considered the first Art Nouveau house, is based on the work of Viollet-le-Duc. The curving stylized vegetal forms that Horta used in turn influenced many others, including the French architect Hector Guimard, who used it in the first Art Nouveau apartment building he designed in Paris and in the entrances he designed for the Paris Metro. He is also considered a precursor of modern architecture for his open floor plans and his innovative use of iron, steel and glass.

Horta's later work moved away from Art Nouveau, and became more geometric and formal, with classical touches, such as columns. He made a highly original use of steel frames and skylights to bring light into the structures, open floor plans, and finely-designed decorative details. His later major works included the Maison du Peuple/Volkshuis (1895–1899), Brussels' Centre for Fine Arts (1923–1929) and Brussels-Central railway station (1913–1952). In 1932, King Albert I conferred on Horta the title of Baron for his services to the field of architecture.

Hector Guimard (French pronunciation: [ɛktɔʁ ɡimaʁ]; 10 March 1867 – 20 May 1942) was a French architect and designer prominent for his Art Nouveau style designs including Paris Métro entrances. He achieved early fame with his design for the Castel Béranger, the first Art Nouveau apartment building in Paris, which was selected in an 1899 competition as one of the best new building facades in the city. He is best known for the glass and iron edicules or canopies, with ornamental Art Nouveau curves, which he designed to cover the entrances of the first stations of the Paris Métro.

Between 1890 and 1930, Guimard designed and built some 50 buildings, in addition to 141 subway entrances for the Paris Métro, as well as numerous pieces of furniture and other decorative works. However, in the 1910s Art Nouveau went out of fashion and by the 1960s most of his works had been demolished, and only two of his original Métro édicules were still in place. Guimard's critical reputation revived in the 1960s, in part due to subsequent acquisitions of his work by the Museum of Modern Art, and art historians have noted the originality and importance of his architectural and decorative works. Guimard was a disciple of Viollet-le-Duc.

The Spire of Notre-Dame de Paris is located above the cross-section of the cathedral's transept. Notre-Dame de Paris has had three timber spires made of oak, known as flèches. The first was built between 1220 and 1230. It eventually became so damaged that it was removed in the late 18th century. The second was put into place by the French architect Eugène Viollet-le-Duc in 1859, and destroyed in a major fire on 15 April 2019. Work to construct a third one started in 2022, and was completed when the new copper rooster wind vane was placed on top of the new spire on 16 December 2023, and the third spire was unveiled on 13 February 2024.

A flèche (French: [flɛʃ] ; French for 'arrow') is the name given to spires in Gothic architecture. In French, the word is applied to any spire, but in English it has the technical meaning of a spirelet or spike on the rooftop of a building. In particular, the spirelets often were built atop the crossings of major churches in mediaeval French Gothic architecture are called flèches.

On the ridge of the roof on top of the crossing (the intersection of the nave and the transepts) of a church, flèches were typically light, delicate, timber-framed constructions with a metallic sheath of lead or copper. They are often richly decorated with architectural and sculptural embellishments: tracery, crockets, and miniature buttresses serve to adorn the flèche.