Villa Adriana had four thermal plants: the Baths with Heliocaminus, the Great Baths, the Small Baths and a small private bath inside the Maritime Theatre.
All the Baths had a “cold” area with a dressing room (Apodyterium), a cold water basin (Frigidarium) and in some cases a large pool also with cold water (Natatio).
Then there was a “hot” area, with a first warm room (Tepidarium), another one with hot water pools (Caldarium) and finally a strongly heated room, a sort of SPA (Sudatio or Laconicum). Inside there were braziers for steam baths, and small basins on pillars that contained cold water (Labrum). In addition there was an outdoor area for exercise, the Palestra (Gymnasium).
These features are present in the Great Baths of Villa Adriana. First of all, there is a large rectangular Palestra (Gymnasium) paved with herringbone bricks (opus spicatum). Then there is the Frigidarium, with two large cold water pools, once cladded in white marble. On the outside there were several heated rooms, among which the circular «sauna" (Sudatio) covered by a dome stands out.
The rooms were heated thanks to a hollow space created under the floors, inside which hot air was circulated. In practice there was a lower floor with a large number of small pillars (all made of bricks) which supported an upper floor. The hot air came from a furnace (praefurnium) located on the same level as the lower floor. It was accessed via a series of subterranean corridors reserved only for slaves, still well preserved, that passed under the Vestibule and reached the Hundred Chambers (Cento Camerelle), where the slaves were housed.
The four thermal plants were all located in the same area of the Villa for a practical reason: the slaves could manage their heating systems using a single subterranean service corridor. Next to the Small Baths is also the Winter Palace, which had a winter heating system even though it was not a thermal building, a sign of great luxury: the emperor lived here in the winter months. By concentrating the thermal and heated buildings in the same area it was easier to supply them with wood to burn in the furnaces.
One last remark: still today someone believes that the Small Baths were reserved for women and the Great Baths for men, a totally incorrect theory repeated as a «copy and paste». The decoration of the two buildings proves in fact that the difference between the users was not their sex, but their social rank.
The Small Baths have a complex architecture, pavements made with precious colored marbles (opus sectile) and marble coverings on the walls, even in the Gymnasium (nothing is left). Therefore they were reserved for the Emperor, and together with the Winter Palace they belonged to his Imperial Residence within the Villa. The same goes for the small miniature baths of the Maritime Theater, which was the personal «villa within the Villa» for the Emperor only.