That's because of the Black Plague, which devastated Europe in the 16th century, that we needed here to extend the small charnel house.
So, in 1432, Nicolas Roussel, vicar of Saint-Maclou, gave a house and a garden in order to extend this cemetery. In the 16th century, they added three galleries then a fourth one in the next century.
Oh, look at those details... so macabre! Beams are decorated with bones and skulls patterns...