The Baths of Diocletian, which are in a very good state of preservation, were constructed between 298 and 306 CE. The baths, which extended over an area of 28 acres and could accommodate up to 3,000 people, were commissioned by the emperor Maximian, who dedicated them to his co-emperor Diocletian.
The principal rooms of the baths were distributed along a central axis. The caldarium, the hot chamber heated using a complex system of air ducts beneath the floors and around the walls, led to the tepidarium, a chamber with an intermediate temperature, and then to the frigidarium, the vast chamber for cold bathing, recognisable today in the Basilica di anta Maria degli Angel e Martiri. There was also the natatio, a vast open-air swimming pool (measuring 4,000 square metres), two enormous gymnasiums arranged symmetrically at either side of the main building, and a series of large halls with various functions.
The Baths of Diocletian were abandoned for approximately 1,000 years until 1561 when, at the behest of Pope Pius IV (r. 1559-65), the surviving structures were transformed into a church and a Cathusian monastery. The Pope entrusted the project to Michelangelo. The church was dedicated to Santa Maria degli Angeli e Martiri, in memory of the many Christians who were believed to have died during the construction of the baths.