The curved walls on the northern side of the theatre were supposed by Nibby to have belonged to another theatre, but are now generally believed to have been a part of a fountain connected with the above-mentioned reservoir. Along the northern side of the reservoir are two parallel walls, which apparently enclosed the street leading to the citadel. The roadway must have been here carried by an arched corridor under the side of the theatre. Near the ancient road from the theatre westwards is a mass of ruins the plan of which cannot be determined, and beyond these, not far from the point where the road divides, and on its right-hand branch, is one of the gates of the city, marked by two fragments of ancient fluted columns which perhaps formed a part of its architecture. Near this are the remains of the ancient north wall of the city, consisting of blocks of peperino of great size more or less regularly laid, and of restorations here and there in reticulated work, partly of the later republic, and partly of more modern times. The pavement of the street is here perfectly preserved, and near the gateway there is a wide space left, probably as a turning place for carts or carriages. [Sidenote: Tank and fountain.] In the walls near this point is a stone doorway leading into a stone water-tank, with a pointed roof formed by overlapping stones, on the same principle as the roof of the so-called Mamertine prison at Rome, the gate of Arpinum and the treasuries of Mycenæ and Orchomenos. The doorway is about ten feet high and five wide, and the tank of the same dimensions. In the interior are three divisions or basins for water, and at the back an aqueduct enters by means of which the water was supplied. At the side of this tank there is a small ancient fountain under the wall which was supplied from the tank by a leaden pipe. An inscription on the fountain records that it was made by the Ædiles Quintus Cælius Latinus, son of Quintus, and Marcus Decumo, by command of the Senate of Tusculum. Not far from the fountain was found the fifteenth milestone from Rome. On the road to Frascati, near the point where the western gate of the city has been supposed to have stood, the remains of an amphitheatre can be discovered. The seats are entirely destroyed, and it is only by the oval shape and by the position of the substruction that the ruins can be recognized as those of an amphitheatre. A round tomb stands a little above the amphitheatre, and beyond this the ruins of a large villa called Scuola di Cicerone cover the side of the hill towards the Alban valley.