Sainte-Chapelle is a royal chapel built in the Gothic style. While the architecture of the building is admirable, the real star is its 1,113 stained glass windows. The windows measure 49 feet (15m) and depict scenes from the Old and New Testaments.
Even on a cloudy day, when the sunlight that comes through the chapel windows is limited, the stained glass is stunning.
Sainte-Chapelle is only about a five-minute walk to Notre Dame Cathedral, so kill two birds with one stone and visit both beauties one right after the other.