This fine structure is built of beautiful white stone, almost marble, in large blocks without mortar, but it had metallic fastenings, which have all disappeared, and left their marks behind them. There are two rows of arches, and above a line of square windows; also a curious stone parapet, with very distinct indications of arrangement for spreading the _velarium_ or _vela_ (the awning). There is only one line of columns, but there was originally a second, and most of the bases of them are still _in situ_. The amphitheatre is built against a rocky mountain, which causes the northern part of the ellipse to be much lower than the outer one. There are numerous passages and substructures, except on the rocky half of the building. There are two principal entrances facing each other, and in a line with them is a trench exactly similar to the one in the Colosseum, and at Capua, &c. These canals for conducting water into the arena can still be seen, and there seems no doubt of its having been used for _naumachia_. There are but few remains of the seats, except a large accumulation of _débris_, and traces of the stairs and _vomitoria_. The whole line of the _podium_ is also perfectly preserved, but no trace of the concentric _euripus_ found in other amphitheatres. The most puzzling parts of the structure are four rectangular towers, which appear to have had no special staircases leading to them from the ground; antiquaries, with all their ingenuity, have not yet given any satisfactory explanation of these objects. They were most probably for the music, as in the circus of Maxentius on the Via Appia, near Rome, where one tower remains at each end of the _carceres_[199]. The amphitheatre at Nîmes, in Aquitaine, still has a wooden floor with trap-doors in it; the present floor is not ancient, but no doubt replaces an old one; there is no staircase, and the only access to the passages below is by a step-ladder, and the arrangement of the substructures is quite different from that in Rome, or at Capua, or Puteoli. There are large masses of stone at intervals to support the floor, and wide passages between them. On two of these massive piers are inscriptions, with the name of the architect, the same inscription repeated twice, in characters of the third century. There are no signs of _naumachia_, or of aqueducts; the seats and the superstructure are more perfect than usual, and have a very fine effect. At Arles the superstructure also is very grand, but there are no substructures under the arena visible. It is partly cut out of the rock, the lower part supported by massive substructures, but no passages in them are visible. Excavations have not been made there, and the doorways appear to be filled up to half their height, as at the Porta Tiburtina in Rome.