This fragile Gothic jewel is home to the most ancient stained-glass windows in Paris. Built by the obsessively pious Louis IX (1226-70), this chapel was constructed in less than three years to house the king's collection of relics acquired from the impoverished emperor of Constantinople at phenomenal expense (and that even in Louis's time were considered of questionable authenticity). Some of these relics have survived and can be seen in the treasury of Notre-Dame, but most were lost during the Revolution.

The building is actually two chapels in one. The plainer first-floor chapel, made gloomy by insensitive mid-19th-century restorations (which could do with restoration themselves), is dedicated to the Virgin Mary, whose statue stands on the pier of the entrance. Today you might be startled to see a souvenir stand taking up most of the space. Up a dark spiral staircase you'll find the king's chapel, which he accessed through the main upstairs door, directly across from his royal bedroom.

Here the famous beauty of Sainte-Chapelle comes alive: instead of walls, all you see are 6,458 square feet of stained glass, delicately supported by painted stonework that seems to disappear in the colorful light streaming through the windows. Deep reds and blues dominate the background glass here, noticeably different from later, lighter medieval styles such as those in Notre-Dame's rose window. The chapel is essentially an enormous magic lantern illuminating the 1,130 figures from the Bible, to create -- as one writer poetically put it -- "the most marvelous colored and moving air ever held within four walls." COST: EUR6.10, joint ticket with Conciergerie EUR8. Métro: Cité.