The frigidarium

The circuit ended with the frigidarium, which was the north-south and east-west pivot in the building's modular layout.

The room was thermically isolated from the rest of the building by four courtyards and thick walls, and was equipped with unheated bathing equipment. In the niches lining the walls were cold-water basins signalled by the presence of water and waste pipes. The exedra in the room's northern side was temporarily converted into an 80-cm deep cold water basin.

After a cold bath, customers would return to the changing rooms, and the circuit ended.

This beautifully proportioned room is famous for its exceptionnal vaulted ceiling that rises to a height of 14.5 metres (one of the highest still standing in the Western Roman Empire), and for the quality of its stone decoration.

The bases of the central vault's diagonals rest on stone consoles featuring bows of large fat-bellied ships loaded with weapons. This décor-exceptional in the Roman world-reflects the power of the Lutetian boatmen's corporation and their role in the building of the baths. It has been suggested that the building could have served as a meeting place for them, but this is far from certain.