At the north end of the Forum are the remains of a very large temple, measuring 35 x 15.5 m. The preserved height is 17 m., the original height may have been over 20. It was built during the reign of Hadrian (c. 120 AD). It has been argued that Hadrian himself and certain people close to the Imperial court contributed to the building of this important temple.
The ruins must always have been visible. Records of the removal of marble from its walls date back to the 15th century. The destruction continued until the early 19th century.
Until the early 20th century it was often stated that this was the temple of Vulcanus, the main deity worshipped in Ostia. Already in the 19th century however, it was suggested that it was the Capitolium, a temple dedicated to the main Roman deities, the Capitoline triad (Jupiter, Juno and Minerva). Capitolia could be found in many cities in the western half of the Empire, following the example of the temple of Jupiter Capitolinus on the Capitoline hill in Rome. The suggestion is supported by the position and size of the temple, but conclusive evidence has never been found. On the other hand: we know from an inscription that the temple of Vulcanus was restored in 112 AD. That date cannot be reconciled with the date of the Capitolium, which is based on accurately dated stamps on several bricks.
In front of the temple was a marble altar, with a frieze of weapons. The temple itself was erected on a high podium, possibly intended as an artificial Capitoline hill. A huge staircase of 21 steps leads to a vestibule, originally surrounded by ten marble columns. A marble frieze with ox-heads and festoons was over the vestibule. The walls of the cella were erected in brick. There is a striking polychrome effect, through the use of yellow bipedales for relieving arches and bonding courses, and of red bricks. Does this indicate that in the Hadrianic period the bricks were not covered? At some point in time the inside and outside of the walls were certainly lined with marble, however: holes for the attachment of the marble decoration (including marble pilasters on the outside) can be seen. Remains of a marble cornice were found.
In the entrance to the cella is a marble threshold, made of one block (6.16 x 1.42 m.). It is only slightly smaller than the threshold of the Pantheon in Rome. The doors were probably made of bronze. The cella had a marble floor, that has now disappeared (it is known from 19th century drawings: squares enclosing rhombs). In the side walls are three niches, the central one semicircular, the flanking ones rectangular. They must have contained statues. Set against the back wall is a podium with a tripartite substructure, accessible via staircases on the sides. One should imagine the cult statues of the Capitoline triad here.
Below the temple, in the podium, is a basement: a row of three rooms, covered by barrel vaults, accessed through a door in the back side of the building. The rooms have floors of opus spicatum. They are lit through slit windows. The function of these rooms is unknown. We are probably not far from the truth however, when we imagine that important records were stored here or the aerarium publicum, the "public treasure".
An inscription mentions an aedituus Capitoli, A. Ostiensis Asclepiades. This temple-keeper was a freed slave of the town, like Q. Ostiensis Felix, temple-keeper of the Temple of Rome and Augustus. The latter temple is the counterpart of the Capitolium, at the south end of the Forum. But the Capitolium also had an effect towards the north. Between the back wall of the Capitolium and the Tiber runs the northern stretch of the Cardo Maximus. The street is a grand avenue, very wide, and flanked on both sides by porticoes. An important point of arrival and departure for sea-faring passengers was presumably located at the spot where the Cardo met the Tiber. We may imagine the departure and arrival on this spot of Emperors and many officials. Those arriving in Ostia would enter the city via the Cardo, to be confronted with the impressive back wall of the Capitolium. After skirting the temple (something only pedestrians could do) they entered an open space: the Forum. It is a well-known architectural effect.
Plan of the Capitolium. After SO I.
Plan of the Capitolium from 1804, showing the pavement of the cella.
From Paschetto 1912, fig. 100.