Sainte-Chapelle

This is Sainte-Chapelle, the French royal chapel. The part you can partly glimpse from the courthouse side is the chapel's north face, while the first thing you see after entering is the south side.

■ The south exterior True to Gothic form, the chapel is marked by straight lines, starting with its tall spire. Famous for its stained glass, it is mostly glass walls from the outside, and the buttresses that support them are the chapel's most distinctive feature. Look at the very top of the buttresses: pointed leaf and bud shapes join into what looks like saw teeth, a common decoration on churches. Combined with the gargoyles, though, they can look like a crown of thorns. That may be all the more fitting since the chapel was built to house the Crown of Thorns relic, now kept at Notre-Dame.

Lower down, a molding band between the first and second floors clearly divides top from bottom. The upper floor, which is Sainte-Chapelle itself, was the royal chapel, and its walls are mostly stained glass. The ground floor was a chapel for the people who worked for the royal household and for the guards, and it has stout, triangular windows.

■ The west façade On the balustrade beneath the large Flamboyant rose window you'll see the fleur-de-lis emblem of the French monarchy, proof that this was a royal chapel. Look for crowns and other royal decoration throughout the building too.

The tympanum of the upper chapel's portal shows the Last Judgment, and the trumeau carries a figure of Christ with one hand raised in blessing. Look at the lower parts on both sides of the door for stories from Genesis, including the creation of Adam and Eve. There used to be a raised walkway linking it to the royal palace, which allowed direct entry here, though it no longer exists. The portal of the lower chapel is dedicated to the Virgin Mary, with a Virgin and Child on the trumeau. It was damaged in the French Revolution and later restored. The tympanum represents the heavenly realm.

■ Inside Let's begin with the main attraction, the upper floor. Sainte-Chapelle's nickname is "the church built of light." On a bright, sunny day, the light pouring through the 15 large stained-glass windows takes your breath away. Read the windows clockwise from the left side of the chapel. They run from Genesis in the Old Testament, through the Resurrection of Christ in the New Testament, to the journey of Louis IX bringing the Crown of Thorns. The rose window depicts the Book of Revelation. Between the windows, each column carries a life-size stone statue of one of Jesus's twelve apostles.

Behind the tall altar at the front of the chapel is where the relic altar originally held the Crown of Thorns and fragments of the Cross. You can see the structure of a reliquary, the most lavish of the medieval period.

If the upper floor overwhelms with its soaring height, the lower floor feels like a cave with its low, rounded rib vaults. The gap between the two floors is as large as the gap in status between those who used them. The lower floor features a deep-blue night sky packed with stars and richly colored red and blue columns. Look for the fleurs-de-lis of the French monarchy and the castle emblem of the family of Louis IX's mother.

■ Special concerts Many European churches hold evening concerts, but Sainte-Chapelle is especially special. As a small Gothic chapel, it produces wonderful resonance from the space alone, without sound equipment. On a summer evening when the sun sets late, hearing music under the glittering stained glass can make you feel devout even if you weren't before. See "Learn more" at the bottom of the in-app script for details and booking links.

■ Visitor tips This is the only paid site on the tour, and you can enter with the Paris Museum Pass. Even with the pass, though, a reservation is not optional but required. There is also a combined ticket with the Conciergerie, where Marie Antoinette was held in her final days, so if you plan to see both, get the combined ticket, which is slightly cheaper. See "Learn more" at the bottom of the in-app script for details and booking links.