This monumental nymphaeum was built at the major intersection of five streets, at the spot where the Decumanus turns towards the south-west. The intersection has been called Bivio del Castrum (Crossroads of the Castrum).

The nymphaeum is flanked by Via della Foce and Via degli Horrea Epagathiana. The masonry (opus latericium) has been dated to c. 350-370 AD.

In the back wall (c. 25 metres long) are three large wall-niches, high up in the wall. The central one is rectangular, the other two are semicircular. In the wall are holes in which marble revetment was fastened. There seems to have been a cistern on the first floor of a room to the north, supported by four corner piers. An enormous trapezoidal basin was set against the back wall. The rear part of the basin seems to have had a roof supported by five brick piers, one in the shape of a plus-sign. On the floor are a few remains of opus spicatum.

Nearby a marble statue of a putto on a dolphin was found (inv. nr. 497), very similar to a statue found in the Macellum (IV,V,2), across the street (inv. nr. 498). Water emerged from the beak of the dolphin.