This chapel was built by order of St. Louis IX to store relics - the crown of thorns. King bought the relic in Venice in 1239, where she was brought from Constantinople. Creator Chapel Pierre de Montero decided to build two churches - one above the other, and both of them were consecrated in 1248. The lower church serves as a high basis for the entire structure, from her rise up huge windows, lancet ending turrets.

The steep slope of the roof is decorated with a light, elegant marble balustrade, and this exquisite architectural element is crowned by an openwork and tall spire height of 75 meters. On both sides of the facade rise two towers with spiers; front facade - portico, over which a large rose window, dating from the XV with scenes of the Apocalypse.

The lower church, a small height - about 7 meters, consists of three naves, but the main nave with side comparison seems huge. Decorative arches in the form of a cloverleaf, supported by graceful columns run along the walls. The apse in the back of the church has a polygonal shape. This part of the chapel intended for the servants, while the luxurious upper chapel, to get into that could be the narrow spiral staircase, visited members of the royal family and their courtiers.

At the top of the church - one huge nave of a width of 17 meters and a height of 20, 5 meters. All the church surrounds a high plinth with marble tracery arcades, interrupted by deep niches. In the third bay there are two niches intended for the king's family. Each pilaster - statues of the Apostles, dating from the XIV century. The design as easy as possible, so as to leave more space for the 15 huge stained glass windows about 15 meters high, that belong to the XIII century, contain scenes of 1134 and cover an area of ​​about 600 square meters. Obvious "flaming" colors are presented to them the Bible and the Gospels.