The Cathédrale Saint-Louis was built between 1743 and 1754 by Jules Hardouin-Mansart de Sagonne (grandson of the architect who built the Hall of Mirrors) as the parish church for the growing town of Versailles. The palace had its own Royal Chapel, but the thousands of tradespeople, servants, and officials who lived in the town needed a church they could actually use.
It became a cathedral only in 1801, when Napoleon reorganised the French church after the Revolution. The interior is restrained compared to the palace a kilometre away — high vaults, good light, and 18th-century furnishings that survived the Revolutionary period relatively intact. The organ is particularly fine.
The cathedral faces the Place Saint-Louis, a quiet square with a Wednesday and Saturday market. This part of town — south of the market and away from the palace — has a more ordinary French character: boulangeries, local cafés, pharmacies. It is Versailles as a town rather than Versailles as a monument.