Sainte-Chapelle is the finest Gothic interior in Paris — a 13th-century royal chapel whose upper floor is encased almost entirely in 1,113 panes of 13th-century stained glass. On a sunny day, the effect is overwhelming: the stone seems to disappear and the whole room glows with colour. King Louis IX built it in just seven years (1244–1248) to house Christ's Crown of Thorns, which he bought from the Latin Emperor of Constantinople for three times the cost of the chapel.
Most visitors go to Notre-Dame and miss Sainte-Chapelle, which is five minutes' walk away on the same island. This is a mistake. The chapel is smaller but architecturally more astonishing, and the crowds are a fraction of those at Notre-Dame. Buy tickets online to avoid queues.