This stretch of opus incertum is Sullan reconstruction when he made a western approach to the Porta Triumphalis to correspond to the one at the east on the arches. This piece of wall is strongly made, and is exactly like a piece of opus incertum wall near the Stabian gate at Pompeii, which Professor Man told me was undoubtedly Sullan.
Marucchi, Guida Arch., p. 19, who is usually a good authority on Præneste, thinks that all the opus quadratum walls were built as surrounding walls for the great sanctuary of Fortuna. But the facts will not bear out his theory. Ovid, Fasti VI, 61-62, III, 92; Preller, Roem. Myth., 2, 191, are interesting in this connection.
I could get no exact measurements of the reservoir, for the water was about knee deep, and I was unable to persuade my guides to venture far from the entrance, but I carried a candle to the walls on both sides and one end.