Virtually in the centre of the Forum, to the south of the Decumanus Maximus, are the remains of a brick circular structure (outside diameter 4.50 m.). It may have been built during the reign of Trajan (98-117 AD), or a bit earlier. A marble cornice runs around the lower part of the outer wall. Marble slabs were probably fastened in holes in the top of the cornice. Inside are six rectangular niches. The northern one (nr. 1) is a blocked entrance. On the floor is a white mosaic.

The structure was regarded as a basin or fountain-house (nymphaeum) until 1962, when Herbert Bloch proposed that it was a compitum. Compita were shrines at crossroads where the gods of the crossroads (Lares Compitales) were worshipped, together with the household gods of the Emperor (Lares Augusti). Bloch suggests that a number of curved inscriptions from 51 AD, found at some distance, belongs to the structure. He also states that the structure cannot be a nymphaeum, because there are no remains of hydraulic mortar, and because it is not clear how water reached and left the fountain. We should be aware that Bloch's remarks about the remains of the shrine, as he himself writes, were not based on autopsy. Being Jewish he emigrated to the United States in 1939.

In 1994 the present author noted that remains of hydraulic mortar (opus signinum) can be seen in the corner between the mosaic floor and the wall. One of the niches has a hole in the back, below a relieving arch (niche 3, which is directly above an old well). The bottom of this niche slopes downwards towards the floor of the structure, starting a little below the present level of the Forum. It was most likely meant for the supply of water. Furthermore, between 1994 and 1996 German archaeologists checked whether the curved inscriptions would fit the outer wall. This was not the case (oral communication, 1996). So the structure seems to have been a nymphaeum after all.

Originally the nyphaeum must have contained four statues, one (in niche nr. 2) opposite a row of three (in niches 4-6). In a fountain-house we may expect statues of nymphs, goddesses related to lakes, rivers and wells. Several inscriptions document the cult of the nymphs in Ostia, and various representations of the sea-nymph Leukothea (appropriate in a harbour city) have been found, for example in the House of the Triclinia (I,XII,1), to the south-east of the Forum. In the nearby House of the Lararium (I,IX,3) a marble well-head was found, probably re-used. It was made for some new well in 197 AD, "after an admonition by Ceres and the nymphs". It is possible that the circular structure hosted a statue of Ceres, opposite three statues of nymphs, and that the well and well-head were originally on the Forum, inspired by the circular structure.

There are a few anomalies in the structure. One would expect the single niche (nr. 2) and the central one of the row of three niches to be on the axes of the Decumanus and Cardo (the axes also of the Forum), but these two niches are placed diagonally toward these axes. The present niche nr. 1 was an entrance. Niche nr. 3 causes a curious asymmetry. In the structure the normal length of wall + niche + wall is 0.65 + 0.88 + 0.65 = c. 2.20. For niche 3 the figures are 0.54 + 0.57 + 1.09 = c. 2.20. Why wasn't this 2.20 divided into the "normal" sections?

Here we may recall a suggestion made by Raissa Calza (1959). Discussing the foundation of the colony, she says about the structure: "A fountain, built later, perhaps recalls the ancient mundus of the Castrum [the military camp that is the oldest Ostia], the sacred well at the centre of the intersection of the two as yet unpaved roads [Cardo and Decumanus], worshipped like a relic". This brings to mind the old well below the niche that supplied water to the fountain. Did it determine the position of niche 3? If the position of niche 3 was fixed, then we can understand the deviating measurements 0.54 + 0.57 + 1.09: these restored regularity to the structure. But this still does not explain the diagonal axes: there could have been one statue in niche 4, opposite statues in 6, 1 and 2. The hypothetical presence of Ceres may now be significant. We know of the mundus Cereris, an entrance to the underworld (from an inscription from Capua mentioning a sacerdos Cerialis mundalis, and from the ancient author Festus). The builders may have wanted Ceres to be close to niche 3, which would explain the diagonal axis.

This is all pretty speculative. One thing is certain however. The circular structure must have been a landmark. Because it is in the centre of the Forum, many people must have gathered and rested here, as many tourists do today.