The church is an example of a Gothic structure clothed in Renaissance detail, and has been attributed to Italian-born architect Domenico da Cortona by Bannister Fletcher. It contains the largest church pipe organ in France. The initial, original architect is unknown. Nicolas Le Mercier (1541-1637), father of Jacques Le Mercier, worked on St. Eustache as a master mason. Seven chapels were original to the plan, two decorated by Mignard and Lafosse. In 1851 Barrias decorated the chapel of St Louis. Among other projects in Paris, Baltard undertook the restoration of many churches, including St Germain-l'Auxerrois, St Eustache and St Leu.