When the germanic tribe of the Alemanni invaded Italy in 270 AD, they were pushed back beyond the borders by the emperor Aurelian, but the decision was made then to fortify the city. Rome had not been enclosed by walls since the days of the Republic, and the urban sprawl had flowed far beyond the lines of the old walls. Rome had not truly needed to be defended as a city and it had been unthinkable that any enemy could threaten the city itself. However, in the wake of such a realisation, the walls were authorised and over 12 miles of high and strong walls were constructed over a mere 8 years, being completed by his successor, Probus, in 279 AD. The walls were renovated and strengthened during the reign of Maxentius in the early 4th century and again by Honorius in the early 5th. They are a truly impressive monument. Following the course of the walls is a big undertaking. Though stretches of the walls have vanished, the southern side of the city makes a tremendous walk. Start at the Porta Maggiore and follow them rough to the Tiber for an idea of just how impressive they are. In their construction, Aurelian made use of any available monument to cut down construction time, and the Amphitheatre Castrense in the Lateran and the Pyramid of Caius Cestius by the Porta San Paolo are primed examples of the reuse of ancient structures.

As with all the Aurelian gates of Rome, this massive fortification was built by 279 AD and underwent alterations and renovations under Maxentius and Honorius. The massive scale of the gate is something of a mystery as the Via Asinaria that passed through it was a fairly unimportant road. Thus the strength must be attributed to the proximity of the Lateran palace, which stood only a few hundred yards away. The gate itself is sealed off from public access, sadly, and is only visible through railings. The modern road system passes through the walls between here and the 16th century Porta San Giovanni.

The gate in the Aurelian walls now known as the Porta San Paolo after the basilica of San Paolo to the south was known in classical times as the Porta Ostiensis. As with all the Aurelian gates, this seems to have been originally constructed with two arches, as is evident from the rear of the gate, which was narrowed down to one arch by Honorius. As with the other gates, this one shows signs of alteration by Honorius, and also by Maxentius before him. The gate stands by the Pyramid of Caius Cestius, opposite the Ostiense station. It is closed to traffic, but very much at the centre of a complex traffic system.

Where the Aurelian walls cross the Via Appia stands the Porta San Sebastiano, named for the church and catacombs away down the road. The ancient name for the gate was, unsurprisingly, the Porta Appia. Originally a double-arched gate, the Emperor Honorious, during his rebuilding, narrowed it to a single arch, and enclosed the curved towers within huge square bastions. At this stage, two wings had been built from the inside, connecting it with the Arch of Drusus just behind to create a small fortress. The rest of the visible work dates to the campaign of Bellisarius in 536 AD. It is one of the most impressive of Rome's gates, and houses the museum of the walls.

The earlier walls of the city (the well-known walls being the Aurelian Walls of 279 AD) enclosed a much smaller area and are traditionally dated to the reign of the Kings of Rome, Servius Tullius specifically in the 6th century BC. Barely any trace remains of the original wall, but in a few places around the city can be seen stretches of a wall constructed some time during the early republic along mostly the same line as the original Servian wall. This likely followed the famous invasion and sack of Rome by the Gauls, which would suggest that by then the original walls had either gone or were in very bad repair. Apart from this impressive stretch just outside the entrance of the Termini station, there are too many fragments of wall around to describe them here. Few are anywhere near the level of preservation of this stretch anyway.

Lying opposite the Circus Maximus, on the lower southern slopes of the Caelian hill, and in the Piazza di Porta Capena, lie some very ruinous walls within overgrown parkland. These are the remains of the Porta Capena in the Servian walls where the Via Appia originally issued from the city. Their form is much different to the servian wall as seen in several spots, but this is likely due to work carried out by either Nero or Domitian, who renovated the gate. At this time the walls were meaningless as the city had grown far beyond them, but the Porta Capena was reused to carry the arches of the Aqua Marcia over the road. In the time of Juvenal's writing, this had acquired the name Arcus Stillans, or 'arch that drips', which reinforces the aqueduct connection.