The lower church was constructed first atop the old ramparts to level a steep slope of six meters and to serve as a foundation for the chevet and the last first traverse of the upper church. It was completed in about 1200. Its arched ceiling is supported by massive pillars and seven arcades, and a wall three meters thick pierced with lancet windows. At one time it apparently served as the master builder's office; the plans of the rose window on the west pignon are etched onto the floor. scratched onto the floor. Today it has a collection of stained glass made between 1391 and 1397 which formerly was installed in the windows of the Sainte-Chapelle chapel constructed by John, Duke of Berry, which was destroyed in 1757.