When Napoleon III decided a new opera house should be built in Paris, Charles Garnier, a practically unknown architect at the time, was selected to design it. Construction of the Opera Garnier began in 1861, during Haussmann’s reconstruction of Paris. The lavish building has a confusing design that is unlike anything else in Paris, but it remains extravagant with its open spaces and wide staircases made this way specifically for those attending to be able to showcase themselves. This opera house was the haven for entertainment and socialization during the late 19th century.
And yes, this is the setting for The Phantom of the Opera.