The aqueduct which supplied the Thermæ of Caracalla crossed the road a few steps before the Aurelian Gate of the city, the Porta Appia (now called the Porta S. Sebastiano), where an arch of travertine, adorned with white marble and pillars of various colours (still standing) was employed to convey the aqueduct over the road. The arch itself is evidently much older than the aqueduct, and has, consequently, been pronounced by antiquaries to be the triumphal arch awarded to Drusus by a decree of the senate, and said to have been erected to him on the Appian Way. It was supported by four columns of African marble, relieved by four niches and an attic above a small pediment; the whole was surmounted by an equestrian statue between two trophies, as shown upon a coin. "The senate likewise decreed for Drusus a triumphal arch of marble, with trophies, over the Appian Way, and gave him the cognomen of Germanicus" (Suetonius, "Claudius," i.). _Passing under, we come to_ THE PORTA APPIA (now Sebastiano), opening on the great highway of ancient Rome, the VIA APPIA. This gate is the finest in the Aurelian walls, and, in its splendid decorations, regard has evidently been paid to the road over which it was built. All the rectangular stones of the substruction are of white marble. It is curious, too, that considerable projections have been left on most of the stones on the right side, whilst the others present a smoothly hewn surface, evidently old material re-used. A fresco painting of the Madonna, said to be of the sixth century, probably the work of a Greek soldier under Belisarius (as the character of the painting is Byzantine), remains in the corridor of Aurelian near this gate. It was over the head of the sentinels in the path and near the third tower on the right side of the gate. The existence of this painting was not known until it was discovered accidentally by Mr. J. H. Parker, C.B., in 1870. _Entrance, first gate on the left, inside the Arch of Drusus._ The gate-house is said to have been built, in the time of King Theodoric, out of the ruins of the Temple of Mars, which stood outside this gate. It was necessary for the Temple of Mars to be outside the gate, and this one was erected when the one outside the Porta Capena became obsolete, being within the Aurelian walls. Behind the right hand wooden gate are a figure of S. Michael and a Gothic inscription cut in the marble, recording the repulse of Louis of Bavaria in 1327. _Descending_ the Hill of Mars, on the left, built into a house, is an unknown tomb. Beyond, we cross the other branch of the Almo. _Upon the left is_ THE SEPTIZONIA OF GETA, the murdered brother of Caracalla. The tomb now only shows a huge mass of concrete. It was named after its shape, and was like the portico erected by Septimius Severus to the Palace of the Cæsars (Spartianus). _On the right, behind the osteria, is the_ TOMB OF PRISCILLA.