Saint-Germain l'Auxerrois is a Gothic church beside the Louvre, long used as the parish church of the French kings. Its layered facade, carved portal, and late medieval interior make it a worthwhile stop when crossing between the Louvre and the Seine.
The church also has a dark place in Paris history: its bell is traditionally linked to the signal for the St. Bartholomew's Day Massacre in 1572. Visit for the architecture, but leave time to take in that history as well.