The Castellum Aquæ Juliæ, or chief reservoir of the Julian Aqueduct, is usually, but erroneously, said to have been the one situated on high ground near the church of S. Maria Maggiore, where the picturesque ruins stand, usually called the “Trophies of Marius,” because those trophies were hung under two of the arches there, until they were removed to the Capitol. A lofty arcade carrying a specus at a high level passes across the valley, from the reservoir near the Porta S. Lorenzo to this point. This arcade is of the first century, built of the fine brickwork usual at that period, and agrees with the time of Frontinus; but the only aqueduct that was high enough to have carried water to that spot was the Anio Novus.
This great reservoir was rebuilt by Alexander Severus, and called a Nymphæum, being given as such on one of his coins. This name is rather a vague one; but it is evident, from some excavations made in 1871, that there were extensive thermæ of the Emperors of the third century near this place.
From this lofty reservoir the distributing channels may be seen branching off in different directions, one going to the Thermæ of Titus, towards the reservoir called the Sette Sale, another going in a different direction.
This Nymphæum (?) is on lower ground than the outer wall, though still on ground so high that the only water that could reach it was that of the Claudia and Anio Novus, of which part of the arcade remains between it and the reservoir on the high bank. From that point the ground descends gradually and gently along the line of the agger of Servius Tullius either way, and still more towards the interior of the city. The aqueduct, therefore, could not pass underground and then emerge in this part. The Porta Esquilina of the inner wall was on the same level, or nearly so. The Porta Tiburtina of the inner wall was near the thermæ of Diocletian, with a gradual and gentle descent to it along the line of the agger. The only possible explanation of the text of Frontinus is that the same names of the gates in the inner wall were applied to those in the outer wall on the same roads.
A piece of leaden pipe, with an inscription upon it, was found at the Porta S. Lorenzo, which gives the names of Dolabella and Silanus as Consuls; this fixes the date at A.D. 10, and shews that the Marcian water was conveyed in leaden pipes at that point in the time of Augustus. The stone specus was carried over the gate, as we see by the remains of it in the wall, and the inscription upon it recording repairs by Augustus. The one upon the leaden pipe is given by Gruter[90].
Another leaden pipe was found by Panvinius on the site of the Prætorian Camp, with an inscription, given by Gruter[91], which records that it conveyed the Marcian water to that point.
Three aqueducts are mentioned in other inscriptions found during the excavations near the railway station in 1869[92]; the three aqueducts intended can hardly be other than the Marcia, Tepula, and Julia. The upper one only was excavated; but if the noble Roman princes who conducted this excavation had dug a little deeper, they would probably have found the other two under the one that they had discovered. These three specus must have been carried along and in the agger of Servius Tullius.
To reach that part of it on the side of which these cippi are found, they probably went along the side of the road that passed on the south side of the Prætorian Camp, on which was the gate called Porta Chiusa. Close to this gate remains of a large reservoir were found in the researches of 1869 on the bank, with the Wall of Rome carried right across it; but this part of the wall is medieval, built of old materials, and it is probable that, originally, the front wall of this reservoir was included as part of the wall of the city, in the same manner as that of the Tepula near the Porta S. Lorenzo. This reservoir, with the Porta Chiusa by the side of it, and the road from it to the agger, at the place where the cippi were found, is now in the garden of the Baron Grazioli.