We passed the Cathedral-Basilica of St. Louis, King of France (commonly called St. Louis Cathedral) several times in our wanderings of the French Quarter, but didn't fully appreciate it until we went inside for Ash Wednesday Mass.
The exterior is well kept and the building is easily seen across Jackson Square from the River Walk. There is a plaza in front of the church that has frequent street performances and artists' displays and it is located adjacent to The Cabildo (old NOLA town hall which now houses the LA State Museum).
From the outside, one would never guess that within this building is housed a majestic interior decorated with gold-leaf murals of the saints and the holy family beginning at the alter's gold pipe organ, spanning the entire length of the cathedral's ceiling and culmulating above the entryway to the church. The floors are tiled with ivory and ebony and each stained glass window is it's own work of art. There are several statues of Jesus, the Holy Family and some saints (including Joan of Arc) standing throughout the sanctuary. The influence of both the Spanish and the French is easily recognized in both the artwork of the church and also the flags displayed near the chandeliers in the main aisle of the sanctuary.
There is a small gift shop that we didn't get a chance to see in the church and there is a website that is very up to date. The church's old convent is located just a few blocks down the road and houses a second gift shop.