The Porta San Paolo (San Paolo Gate) is one of the southern gates in the 3rd-century Aurelian Walls of Rome, Italy. . . . The original name of the gate was Porta Ostiensis, because it was located of the beginning of via Ostiense, the road that connected Rome and Ostia where functioned as its main gate. Via Ostiense was an important arterial road, as evidenced by the fact that upon entering the gate of the same name, the road split, with one direction leading to the famous Emporium, the great market of Rome. The gatehouse is flanked by two cylindrical towers, and has two entrances, which had been covered by a second, single-opening gate, built in front of the first by the Byzantine general Belisarius (530s–540s). The structure is due to Maxentius, in the 4th century, but the two towers were heightened by Honorius.
St. Paul’s Gate (Porta San Paolo, in Italian) is part of the complex of the Aurelian Walls, built by order of the Emperor Aurelian in 275 CE, and presents itself as one of the best preserved city-gates in the whole circuit of walls. The current name came into use during the Middle Ages because of its proximity to the Basilica of St.Paul outside the Walls, which could be reached by means of the Ostian way, beginning its course, leading to Ostia (hence its name) precisely from this gate.
In its original phase it consisted of two twin arches with two semicircular towers. Restoration and refurbishment works were carried out under the rule of the emperors Maxentius (306-312 CE) and Honorius (401-403 CE) so as to change its aspect, that currently shows an entrance arch placed before the original two-arched gateway. The Gate underwent several modifications also in the next centuries. During the Greek-Gothic war, in 594 CE, the Goths of Totila entered Rome through it. The Gate now appears isolated, but it was originally connected to that stretch of the Aurelian Walls descending from the hill of San Saba as far as the Pyramid of Caius Cestius. It was isolated from the walls as early as in 1920 to facilitate the traffic flow in the square, whilst the stretch of the wall that connected it to the Pyramid was destroyed during the bombing in 1943.
Imperium Romanum: Porta Ostiensis (Porte Aureliane) is a useful page but it’s in Italian. Google Translate’s attempt.
Gate of San Sebastian with Arch of Drusus
The Porta San Sebastiano is the largest and one of the best-preserved gates passing through the Aurelian Walls in Rome (Italy). Originally known as the Porta Appia, the gate sat astride the Appian Way, the regina viarum (queen of the roads), which originated at the Porta Capena in the Servian Wall. . . .The original structure was constructed by Aurelian ca. AD 275 and included a double-arched opening surmounted by bow windows and two semi-cylindrical towers. The façade was faced with travertine. After a later restoration, the towers were enlarged, increased, and linked, through two parallel walls, to the preexisting Arch of Drusus.
Wikipedia (Porta San Sebastiano)
The Arch of Drusus is an ancient arch in Rome, Italy, close to the First Mile of the Appian Way and next to the Porta San Sebastiano. . . . Only the central part of this arch is now standing, but it was originally triple, or at least with projections on each side, although never finished. It is built of travertine, faced with marble, and on each side of the archway are columns of Numidian marble with white marble bases. The archway is 7.21 metres high. The Aqua Antoniniana, the branch of the Aqua Marcia, ran over this arch, but the brick-faced concrete that is visible on the top seems to belong to a later period. The arch may possibly be the Arch of Trajan.
Wikipedia (Arch of Drusus)
The real name of this monumental gate, one of the largest and best conserved in the Aurelian Walls, was Appia, from the name of the important arterial road which it opened out onto. In the Middle Ages the name was corrupted into Daccia and Dazza, over which the name Porta S. Sebastiano eventually prevailed, in honour of the Christian martyr buried in the church on the Via Appia not far from the walls. The present appearance of the Gate is the result of many architectural transformations, which succeeded each other through the course of the centuries, and which can be divided into five periods from the antique period onwards:
Museo delle Mura