The Gothic cathedral at the heart of Vieux Lyon was built between the 12th and 15th centuries and served as the site of several royal marriages, including Henry IV's marriage to Marie de Medici in 1600. The exterior is decorated with hundreds of small medallions carved into the façade — a Romanesque tradition continued through the Gothic construction. The striking interior has good stained glass in the choir chapels.
The real curiosity is the astronomical clock in the north transept, a 14th-century mechanism that, on the hour at noon, 2pm, 3pm, and 4pm, triggers an elaborate automaton sequence. It's worth timing a visit to catch it. The cathedral is surrounded by the most atmospheric part of Vieux Lyon, and the Place Saint-Jean outside is a good starting point for exploring the neighbourhood.