Sainte-Chapelle

Louis IX built this 13th-century chapel specifically to house Christ's Crown of Thorns, creating what amounts to a reliquary the size of a building. The upper chapel's 15-meter stained glass windows tell the entire Biblical story from Genesis to Christ's resurrection in 1,113 individual scenes - you can spend an hour just decoding the stories in the Rose Window alone. When afternoon sun hits the western windows, the entire space glows like the inside of a sapphire.

You'll enter through the lower chapel, which served the palace staff and feels surprisingly dark and cramped. The narrow spiral staircase deposits you into the upper chapel - the contrast is genuinely startling. The walls disappear into towers of colored glass, and your eyes need a moment to adjust to the kaleidoscope effect. Most people photograph frantically for five minutes then stand transfixed.

The audio guide is actually worth it here - without context, the windows look beautiful but meaningless. Avoid the gift shop entirely and don't bother with the lower chapel beyond a quick glance. The magic happens upstairs, and you'll know within thirty seconds whether the light is good enough that day to make the visit worthwhile.