Saint Eustache Cathedral has its origins in a church dedicated to Saint Agnes. This was true until the 16th century when, in 1532, Jean de la Barre, the mayor of Paris began work on what is now Saint Eustache Cathedral. The reason for the beginning of this work was because it had become necessary to enlarge the town square. The work was discontinued during the religious wars, and was not begun again until 1624.
The church is 105 meters long by 43 meters. The exterior of the church is very ornate and beautiful and has a gothic architecture like Notre Dame. Its principal characteristics are its pillars and arches and its choir. The church distinguishes itself by the variety of its architecture: The interior is gothic, but the decor itself is done in Renaissance style. Several famous people are buried within its walls, such as Molière, Richelieu, and the marquise de Pompadour. On the exterior of the church, one finds numerous gargoyles, which makes the architecture particularly admirable. (paris pages.org)