This chapel was built by the order of St. Louis IX to store relics - the crown of Thorns. The king bought this relic in Venice in 1239, where it was brought from Constantinople. The Creator of the chapel of Pierre de Montereau decided to build two churches, one above the other, and they were both consecrated in 1248. The lower Church serves as a high base for the entire structure, from her rise up the enormous Windows ending in pointed turrets.

The steep slope of the roof is decorated with light and elegant marble balustrade, and this exquisite element of architecture crowned with a delicate, soaring spire height of 75 meters. On both sides of the façade the two towers with spires; in front is a portico, over which a large rose-window, Dating from the XV with scenes of the Apocalypse.

The lower Church, a small height of about 7 meters, consists of three naves, but the main nave compared with side looks huge. Decorative arches in the shape of a Shamrock, based on graceful columns move along the walls. The apse end of the Church has a polygonal shape. This part of the chapel was intended for the servants, while the luxurious upper chapel, access was via a narrow spiral staircase, visited members of the Royal family and their courtiers.

The upper Church is one huge nave a width of 17 meters and a height of 20.5 meters. The Church is encircled by a high plinth with marble tracery arcades, interrupted deep niches. In the third flight are two niches, destined for the king of his family. Each pilaster - statues of the apostles, Dating from the XIV century. Design as easy as possible, so as to leave more space for 15 huge stained glass Windows with a height of about 15 meters, which belong to the XIII century, contain 1134 scenes and cover an area of about 600 square meters. Bright "flame" colors presented to them biblical and Evangelical scenes.