Sainte-Chapelle was built to hold a single object: the Crown of Thorns, which cost King Louis IX more than three times the price of the chapel that shelters it. Consecrated in 1248 after only a few years of work, the whole building is essentially an oversized case for that one relic, made of stone and glass and raised beside the royal palace on the Île de la Cité. Its builders pushed Gothic engineering to a limit — the upper walls are almost entirely glass, held by stone ribs so slender the structure seems to defy its own weight.
You enter through a low, dim lower chapel, its vault painted deep blue and scattered with golden stars, then climb a tight spiral stair — and the space bursts into colour. Fifteen windows rise some fifteen metres, roughly eleven hundred scenes running from Genesis to the Resurrection, two-thirds of the glass still medieval. On a bright afternoon the light turns the floor into a shifting mosaic of red and blue. The relics are long gone, scattered in the Revolution, and the chapel now stands inside the city's law courts — but the glass remains its reason to come.