The caldarium, tepidarium and apodyterium were added in the late second century BC to the eastern side of the palaestra. The baths were enlarged one last time, when the circular sweating room was added, around 80 BC. .
Ultimately, the baths were transformed into huge social centers much like present day health spas. Common bath complexes varied little from the "traditional-, represented by the Stabian Baths. The most elaborate baths contained every imaginable convenience. In addition to the baths, sweat rooms, sports grounds, latrine and dressing rooms; they were equipped with swimming pool, running track, gardens, tanning areas, shops and even libraries. Another outstanding feature of the changed grounds was the addition of the perimeter wall. The first baths to show evidence of a perimeter wall, are the Baths of Trajan. These were the first of the giant baths to be built, 104-109 AD. The baths were joined to the perimeter wall on the northeastern fazade of the building. One of the most impressive, famous and by far the largest of the giant baths is the Baths of Diocletian (ill. 3); it is visibly different from the Baths of Trajan, as well as other impressive baths of Rome (i.e., the Baths of Carcalla), in that it had a perimeter wall adjacent to but not touching its actual structure. The second substantial difference is the massive natatio, which is defined as a swimming pool, "a huge sheet of water which covered three and a quarter times the area of that in the Baths of Carcalla."" The illustration of Diocletian's baths shows the swimming pool to the left and the great hall where the men and women are dwarfed by its size. In order to grasp the size of the baths, today it houses two churches and a large museum. .
The hot humid conditions that the baths required greatly influenced the architecture and design of these buildings especially their concrete-vaulted structure.