Let us now trace this aqueduct _backwards_ from Rome, in that part near the city. At the Nymphæum, called the fountain of Egeria, it turns at a sharp angle up to another small _castellum aquæ_, close to the church of S. Urbano, which is usually mistaken for a tomb; but the walls are lined with _opus signinum_ or _coccio pisto_, the invariable test of an aqueduct. At this point it turns again towards another fine _castellum aquæ_, at the end of the Circus of Maxentius and of his son Romulus. It comes on there from the head of the valley of the Caffarella, and to that point from the villa of the Quintilii in the Via Appia Antiqua. Thus far we have traced its course _backwards_ from the _thermæ_; in order to identify it more clearly, we will now go to the sources, and bring it down to the same point to which we have traced it from the _thermæ_. The sources or springs are on the side of the hill of Marino, below Grotta Ferrata, in the same swampy district as those of the river Almo, and of the old aqueducts of the Tepula and the Julia. Several springs are collected into a central reservoir; and, as is usual with other aqueducts, they are then brought into a _specus_, at first underground to the foot of the hill, then upon an arcade, of which there are considerable remains at the Torre di Mezza, Via di Albano, about seven miles from Rome. This fine arcade of the third century goes on to the villa of the Quintilii, and was no doubt built under Aurelius Commodus, after he had obtained possession of that great and fine villa. In various parts of that great building are remains of reservoirs for this aqueduct, and of baths connected with them[183]. About half-a-mile nearer to Rome is, at an angle, as usual, another large reservoir, which has been turned into a farm-house; and the appearance of a medieval chapel or church has been given to it, by building a tower at one end. Possibly it was used for that purpose in the Middle Ages. From thence to the head of the valley of the Caffarella is but a short distance. Where the _specus_ has been carried on an arcade above ground, it has been destroyed as a quarry for the good old bricks; where the ground is higher and the _specus_ is underground, it has been traced in parts only.