To the west of the Basilica, accessible from the Decumanus, are the remains of a huge round temple with a forecourt. Not much has been preserved of this building, but its importance is obvious.
An analysis of the masonry and the architectural decoration shows that the temple was built in the second quarter of the third century AD. It has been assigned to the reign of Alexander Severus (222-235 AD) or Gordianus III (238-244 AD). It is the last major temple that was built in Ostia.
The Round Temple was excavated in the years 1802-1804. A report was published in 1805. For some time it was called a bath (lavacrum). C.C. Briggs published an article about the temple in 1930, but only in 2004 a thorough publication appeared, by A.-K. Rieger.
Before discussing the god(s) to which the temple was dedicated, let's have a look at the remains. The front wall has disappeared completely. The complex is at a higher level than the street, and there must have been a staircase. The forecourt (41 x 35 m.) replaced a square with marble colonnades and a marble pavement, belonging to an earlier building. The older square could be reached from the Basilica (to the east) through five passages. When the new square had been built, one passage remained. Almost exactly opposite this passage, in the west wall, another opening led to the Vico del Tempio Rotondo. Later it was blocked by a niche, perhaps a fountain. The new square was paved with a black mosaic and reused marble slabs, and probably also had a porticus. Niches decorated with marble were built in front of the east and west walls of the new square.
The temple is on a podium, 3.80 m. higher than the square. The wide, rectangular vestibule of the temple was reached by a wide, marble staircase with eleven treads, between two large niches. At the front of the vestibule were ten marble columns. The round cella has a diameter of 18.30 m. (less than half the diameter of the Pantheon in Rome). The walls are two metres thick. To the west of the entrance is a spiral staircase, that led to the cupola. It spirals upwards around a thick travertine column. There is an identical, empty room to the east of the entrance.
In the cella are seven niches. Three are rectangular and may have contained up to three statues each. Four semicircular niches contained four more statues. Between the niches are the bases of eight columns, that supported the dome. Remains of a channel in the cella indicate, that there was an opening in the centre of the dome, as in the Pantheon. Below the rear niche, in the substructure, is a small room with a well and a staircase. In the substructure of the cella parts of travertine columns and capitals can be seen. These were taken from baths to the south (IV,IV,8), like the column in the spiral staircase.
The scale and expensiveness of the building suggest that the Emperor himself was involved with the construction. Meiggs has suggested that it was Gordianus III. A colossal head of the Emperor that was found in the Piccolo Mercato (I,VIII,1) may come from the temple. His father may have lived in Ostia (a sarcophagus made for him was found not far from the city). Inscriptions found in the temple mention the wife and a son of Gordianus.
Unfortunately it is difficult to establish where the sculptures unearthed during the years 1802-1804 were found exactly. To the temple can also be ascribed a colossal head of Alexander Severus, found in the Piccolo Mercato like the portrait of Gordianus. In the area portraits of Trajan, Hadrian, Antoninus Pius and Marcus Aurelius were unearthed, and a few remains of the remainder of the statues (it is curious that the portraits were found, and that the rest of the statues has largely disappeared). It has been suggested that these were found in the Basilica. It seems possible however, that they originate from or were unearthed in the Round Temple. In that case they belonged to an older Augusteum, and were later placed in the third century temple. The statues were 3 to 3.50 m. high.
The finds and, of course, the resemblance to the Pantheon in Rome, have led to the identification of the Round Temple as a temple dedicated to all official Roman gods, or a temple dedicated to deified Emperors (Augusteum). The emphasis must have been on the Imperial cult. Coarelli has pointed out that, whereas the temple in Rome was dedicated to all gods (in view of its name), it was nevertheless primarily an Augusteum, in which the Emperor presided over the gods as primus inter pares.
The cult of the deified Emperors in Ostia was much older than the period of construction of the Round Temple. Priests (flamines) are documented of the deified Vespasian, Titus, Hadrian, Antoninus Pius, Marcus Aurelius, Pertinax and Septimius Severus. We also hear of a flamen divorum, a priest of all deified Emperors. However, the number of temples dedicated to deified Emperors identified so far is surprisingly small (excepting temples and shrines in guild-seats and in the Barracks of the Fire Brigade). An inscription documents a Templum Divorum, built during the reign of Antoninus Pius. It is conceivable that the latter temple was on the spot of the Round Temple, and that the cult of the deified Emperors was situated in a few, rather than in many temples.
Plan of the temple. After SO I.
Plan of the temple from 1805,
by Guattani and Holl. South is up