West façade of Saint-Denis, before the dismantling of the north tower (c. 1844 – 1845)
The Basilica of Saint-Denis (French: Basilique royale de Saint-Denis, or simply Basilique Saint-Denis) is a large medieval abbey church in the city of Saint-Denis, now a northern suburb of Paris. The building is of singular importance historically and architecturally as its choir, completed in 1144, shows the first use of all of the elements of Gothic architecture.
The site originated as a Gallo-Roman cemetery in late Roman times. The archeological remains still lie beneath the cathedral; the people buried there seem to have had a faith that was a mix of Christian and pre-Christian beliefs and practices. Around 475 St. Genevieve purchased some land and built Saint-Denys de la Chapelle. In 636 on the orders of Dagobert I the relics of Saint Denis, a patron saint of France, were reinterred in the basilica. The relics of St-Denis, which had been transferred to the parish church of the town in 1795, were brought back again to the abbey in 1819.
The basilica became a place of pilgrimage and the burial place of the French Kings with nearly every king from the 10th to the 18th centuries being buried there, as well as many from previous centuries. (It was not used for the coronations of kings, that function being reserved for the Cathedral of Reims; however, French Queens were commonly crowned there.) "Saint-Denis" soon became the abbey church of a growing monastic complex.
In the 12th century the Abbot Suger rebuilt portions of the abbey church using innovative structural and decorative features. In doing so, he is said to have created the first truly Gothic building. The basilica's 13th-century nave is the prototype for the Rayonnant Gothic style, and provided an architectural model for many medieval cathedrals and abbeys of northern France, Germany, England and a great many other countries.
The 86m high spire, dismantled in the nineteenth century will be rebuilt. The project, initiated more than thirty years ago, will begin in May 2020. It should take about eleven years and cost 28 million euros. 
Saint Denis, a patron saint of France, became the first bishop of Paris. He was decapitated on the hill of Montmartre in the mid-third century with two of his followers, and is said to have subsequently carried his head to the site of the current church, indicating where he wanted to be buried. A martyrium was erected on the site of his grave, which became a famous place of pilgrimage during the fifth and sixth centuries.