The Cathedral is beautiful you can see the various time periods of work right away with the windows. We happened in on a Sunday morning and caught the last mass of the day by chance. It was beautiful, a lone celloist playing on a small stage facing the alter. The sun happened thru a window and lit him up like a spotlight on a soloist in an orchestra. Both the priest and the cantor had wonderful singing voices. The female cantor's voice was operatic and the acoustics were fabulous. Even though the mass was in french it was still easy for us to follow. We noticed quite a few none catholics enjoying it as well because the music and singing were so beautiful. As soon as mass was over the rest of the cathedral was opened to tour and photograph. Wonderful pieces of art. Definately a must see no matter what faith you practice. Small 2 euro fee to see the crypts otherwise the visit is free.