While we were waiting in line for Sainte-Chapelle, we were waiting in front of the Conciergerie. Not knowing what it was, we looked it up in our handy Lonely Planet book. We learned it was originally a royal residence in the 6th century and lasted as such until the end of the 14th century. In 1391 the building was converted to a prison that held both common prisoners and political prisoners. At this time prisoners were put in cells based on the "rent" they could pay for them thus rich prisoners were able to afford single cells with enough room for a bed and a desk. The prison's most famous prisoner was Marie-Antoinette.
This Chapel is where Marie Antoinette's cell was thought to be located.