Construction of Notre-Dame began in 1163 under Bishop Maurice de Sully and continued for nearly two centuries, with major campaigns led by Jean de Chelles and Pierre de Montreuil. It is the textbook example of the transition from Early French Gothic to Rayonnant Gothic, and it is one of the first cathedrals to integrate the flying buttress as a planned structural element rather than as a corrective afterthought.
The cathedral is laid out as a three-aisle basilican plan with a non-projecting transept and a double ambulatory wrapping the choir. The west facade is composed on a strict three-by-three grid: three portals at ground level, a horizontal gallery of kings above them, the great rose window at the centre of the second tier, and a paired arcaded gallery beneath the twin towers at the top. The proportional system follows the medieval method of composition by squares, known as ad quadratum, in which the principal divisions of the facade are derived from a single governing square. The towers are flat-topped rather than spired, a habit inherited from Norman precedent. Internally, the elevation was originally four storeys (arcade, tribune, oculus, clerestory), but it was simplified during construction to a three-storey scheme (arcade, oculus, clerestory) once the structural redundancy of the tribune was recognised. The flying buttresses, an integral element rather than an addition, carry the thrust of the rib-vaulted nave outward across the side aisles to slender external piers, freeing the wall plane to dissolve into stained-glass clerestory and rose windows. The two great roses, in the north and south transepts and dated to around 1260, are among the largest of their period.
The nineteenth-century restoration by Eugène Viollet-le-Duc, carried out between 1844 and 1864, is inseparable from the building we know today. He added the spire, since lost, the chimeras of the upper galleries, and much of the sculptural programme that visitors photograph as medieval. After the fire of April 2019, the rebuilding deliberately reconstructed Viollet-le-Duc’s spire rather than commissioning a contemporary replacement. This was an unusually conservative choice in twenty-first-century preservation practice, and a statement about which Paris is being preserved.