Le déambulatoire

The ambulatory of a church or cathedral is the pathway around the choir and altar on the building's east end. Often, as in this case, the ambulatory is lined with semi-circular apsidal chapels that protrude from the building.

Not a whole lot of religious fervor on this day. Notice the stone floor.

Without going into the details, the ambulatory provides a path for liturgical processions and for pilgrims, separating them from the main holy action pomp and circumstance in the choir and at the altar. Wikipedia says, cet espace de circulation canalisait la ferveur religieuse tout en protégeant de la foule le bon déroulement des offices (this circulation space channels religious fervor, protecting the orderly celebration of mass and other daily prayer ceremonies from the crowd).  I don't know this stuff, I only report what I read on the internet.