Significant plot details follow.
Read at your own risk.

Notre Dame Catherdral is a location used in Rhythm Thief and the Emperor's Treasure. It can be accessed from Petit Pont.

Raphael and Marie first access this area in Chapter 2 when a clue written on the side of Marie's violin leads them there. The stained glass window at the back of the cathedral lights up when the song Moon Princess is played, and when its panels are touched in the right order, the Dragon Crown appears.

Hubert, a priest, usually stands outside of the cathedral. Bruno is first met here as well.