Calling it "thermal baths" is reductive, calling it "Rotonda" is incorrect. The analysis of this monument begins from its own name that carries with it some imprecisions. Indeed, the structure was used as thermal baths only for a short lapse of time: from the I century A.d. to the VI century a.d. In 584 it began instead a long period in which the Rotunda was firstly transformed into a simple church and it is also possible that the Rotonda had been the first cathedral of Catania.
The name "Rotonda", which means circular, dates back to the period when it was thought that the circular structure had been constructed before the squared walls that enclose the first structure. However, this theory was proved wrong, as it has been demostrated that, without the external walls, the Rotonda would have fall.
The "Rotonda" was part of a wider complex of thermal baths called "Grandi Terme" by the prince of Biscari, and they went from Piazza Dante right next to the Roman Theatre.
It has been found that these Thermal baths go back to the I century a.d., went trough a phase of enrichment in the III century a.d. and were then abandoned in the VI century a.d. They consist of nine rooms that cannot surely been identified: it is plausible that the wide central room, that has the shape of an apsis, was the frigidarium, instead, the place that was after transformed into a church, was the calidarium.
Even if they were public thermal baths, they were luxurious and enriched, as we can see from the remaining fine marble and mosaic on the pavement.
The thermal baths were constructed on a series of "ipocausti", that are gaps between two pavements, supported by small columns called "suspensurae", in which hot air passed in order to heat all the room. This system was also used in the walls, but using terracotta tubes.
One of the last examples in Catania of ogival arch gives access to the interiors of the Rotonda.
During the XII century the area around the Church became a necropoli