Dates to c. mid-4th century CE. The baths were ordered by Constantine to give Aquileia monumental thermal structure suiting its status as an imperial capital. They covered an area of about two hectares, rivalling in size the Baths of Caracalla in Rome. The baths were in use through the 5th century due to refurbishing and survived even the sack by Atilla the Hun in 452. In the 6th and 7th centuries modest habitations and a sepulchral area were built in the ruins of the baths. Up to the 13th century the walls of the baths were standing, after which they began to be dismantled and the area was turned over to agricultural use. The archaeological exploration of the area was carried out in 1922-23, 1961, 1981-87 and in 2002. The baths complex is on the model of the roman imperial baths complexes. Two large covered aula (20X30) were located north and south of the frigidarium. They were decorated with elaborate polychrome mosaics, with the north aula having scenes in rectangles and squares from marine mythology and the south aula scenes of hunting. The north and south ends of each aula terminated in a wide rectangular panel which figured four busts of athletes enclosed in squares or octagons.
