This 13th-century royal chapel was built in 1242-48 by King Louis IX to house his religious relics brought back from the Holy Land. It is said that it once housed Christ's crown of thorns. Originally part of the royal palace complex on the Ile de la Cité, it is now incorporated into the administrative complex called La Conciergerie. That's part of the reason why you have to go through a security check by the gendarmerie upon entering. The stunning Gothic structure boasts two levels, including an upper section that's awash in light from gorgeous stained-glass windows, depicting scenes from the Old Testament and the Passion of Christ.The chapel is a perfect example of the Gothic architectural style called Rayonnant and its "jewel-box" structure, along with its acoustics within, are renowned worldwide. It has been a national historical monument since 1862. Not to be missed. METRO: Cité