Achillian Thermae are underground Thermae dating to the fourth-fifth century of which remains just a small portion visible under Duomo Square. Thermal complex is leading passing by a corridor with a barrel vault. The plant name is derived from Greek inscription on marble slab dating from the first half of the fifth century. The existence of the building under Constantine I is postulated based on the re-use, in the Cathedral, a group of capitals of the period that could come from this building. In 1856, they found the ruins attributed to the same part of this building, possibly relevant to a calidarium. After the work of paving of the square (2004-2006), the building was re-opened to the public and to the realization of events. Original plant is a central chamber, whose vaulted ceiling is supported by four quadrangular pillars, which opens on to a series of parallel basins to forming part of a complex system of canalization, drainage and water filtration. In ancient times, the floors were marble, while the walls and ceiling were in stucco inspired by the world of harvest.