The thermal building of Cefalà Diana is located at the foot of Mount Chiarastella, just up against a spur of rock from which a hot water source gushed (35.8°-38°).
It is part of the architectural typology of ḥamma Islamic (thermal bath that used the natural heat of water and that needed only the presence of swimming pools for diving) and it is the only monument of this type well preserved in the history of Sicilian architecture.
It looks like a compact parallelepiped characterized on the top of sides W, N and E by a cufici characters inscription, carved in blocks of calcarenite and painted in white on a red background, between slightly jutting frames decorated with vegetable motifs.
The external vestments of the structure are of shapeless stone of considerable thickness (1.60 m) put together with mortar and they show uneven construction characteristics: below the epigraphic band, they are provided with large brick listings and cantonals; above, they show cantonals in sandstone blocks.
The main entrance is located on the northern part, while two are specularly in the middle of the longitudinal walls and another one at the southwest corner. The last one and the one on the east side have been walled up and transformed into windows in the course of time.
The inside space of the building is divided into two parts by a three-arched wall which lie on marble columns surmounted by pulvini and terracotta capitals.
The southern area includes a small vaulted room that includes the spring from which hot water gushed and a first tank ; the northern area, longer than the previous one, includes three tanks covered with large bricks.
The initial exploitation of the spring dates back to the first half of the 10th century:it was used, perhaps,also for agricultural irrigation, , but nothing shows the presence of a stable settlement or the creation of any structure.
The construction of a ḥamma with an entrance to SW, with the same extension of the current building, covered and architecturally defined in its essential lines according to a conception and an organization of the paths transformed during the following centuries dates back to the second half of the 10th century and the beginning of the 11th century.
The vaulted room that houses the spring is pertinent to this Islamic building, while the southern area had a single large swimming pool.
During the Norman period, the bathroom was reconfigured by Roger II (1140-1141), whose commission is known through the reading of the epigraph that decorates the exterior of the building and whose insertion is the most evident sign of the intervention of monumentalization of the structure.
The decisive point of the transformations carried out by the Norman king is the opening of a new majestic entrance at the centre of the N side, which had not been affected by access routes until then, indicating a new conception of the inside of the building according to a longitudinal axiality emphasized by the construction of the wall on three arches.
Unlike what was supposed to happen in the Islamic bath, it is privileged a longitudinal directionality with the northern portal on the one hand and on the other one the vaulted compartment that housed the pre-existing Islamic spring, incorporated in this new arrangement, but visually screened by the three-arched wall, perfectly centered in relation to the new internal path. Even the system of distribution of water taken from the South, undergoes a clear change.
The general organization of the site, however, does not present any continuity between the Islamic period and the Norman one.
After a period of slow state of neglect during the second half of the thirteenth century, at the beginning of the fourteenth century the site of Cefalà Diana baths has a new redefinition with the installation of a fondaco, attested by documents from the end of the fourteenth century, and of a horizontal wheel mill.
The life of the fondaco is connected, at least for the whole XV century, to the public road Panormi and to Cefalà Castle.
In the middle of the eighteenth century Niccolò Diana, Baron of Cefalà and founder of the actual town of Cefalà Diana, creates the current pool located to the south of the wall, divides into three tanks the previous large only pool in the northern sector, builds the vault made of calcarenite, probably for the failure of the previous brick vault, reorganizes the water collection and distribution system and opens two new entrances at the center of the east and west sides of the building.
The most recent phase of renovation of the inside of the thermal bath dates back to the nineteenth century, when the tanks are restricted, raised and equipped with quadrangular small bathtub and a large channel that conveys the most water from the source to the large feeding tank of the mill that is built to the north-east of the complex
The actual arrangement of both areas and of the water distribution system is therefore the result of a series of interventions that have marked the long history of the building and that have transformed the original plant.