But the Aurelian walls are just the latest fortifications of the city. First came the mythical pomerium of Romulus. According to the story, Romulus used a plow to mark off the boundaries of his fortress on the Capitoline Hill, carefully lifting the plow at the places where the gates would go. It was this plowed boundary that Remus overleapt to start the fight that ended in his own death at the hand of Romulus. The word pomerium eventually came to mean the "city limits", which had ritual, religious, and military significance.
Establishing city boundaries by the plowing ritual was part of the "Etruscan method" that Rome consciously adopted from their northern neighbors -- the other major part was laying out the grid of streets on a carefully determined north-south axis, but that was used later for new towns. The use of the plow is thought by modern archeologists to have been a ritual enactment of entrenchment: a plow would mark the line and then a ditch or fosse would be dug around the hilltop settlement. The dirt thrown up became a mound or agger on the inner side of the ditch. Wooden revetments, which might have originally been used to prevent erosion of the mound, soon became palisades, a feature of this kind of defensive works. Digs on several of the Roman hills have yielded remains of such fortified village boundaries dating from the appropriate period.
The most important of the "agger and fosse" defensive works are usually ascribed by scholars to Servius Tullius, who, according to tradition, ruled as Rome's sixth king from 578 to 534 BC. Servius's ditch probably ran only across the upland ends of the Quirinal, Viminal, and Opian ridges (called "hills" by the Romans). Natural slopes were relied upon to defend other parts of Servius's town, and pre-existing separate defensive works surrounded the Palatine and Capitoline areas.
Servius's name was later attached to the complete circuit of stone walls that were built some time after 390 AD. These "Servian" Walls pretty closely followed the line of the Servian ditch, but were extended to include parts of the Caelian and Aventine ridges, and the detached Palatine and Capitoline hills. Like most city walls throughout history, the Servian walls followed natural topography to the extent possible and were either built above or in front of defensible slopes and crossed valleys only to complete the circuit. The date of the Servian walls is fairly clear because they were built predominantly of greyish-yellow tuffa stone from the Grotta Oscura quarries in Veii, a type of stone not available in Rome until after it had taken Veii in the first quarter of the 4th century BC. The Servian walls can also be seen as an attempt to close the barn door after the horses were already out -- they were built after the sack of Rome by the Gauls, to try to prevent a repetition of that terrible event of 390. Remains of the Servian wall can be seen in several parts of modern Rome -- on the western side of the Aventine, near the Auditorium of Maecenas on the Opian, and, most easily, right in front of the Termini Railroad Station.
In fact, there was no need for any further work on the walls for some time. Being sacked by the Gauls left the Romans really angry and determined. Rome went on the offensive, and it was other, mostly Gallic, cities that needed ditches, palisades and walls for protection against the Romans. (Go to the following Internet address for a short history of how the Gauls were conquered and absorbed: http://www.britannica.com/search?query=gauls&ct=&fuzzy=N.)
The city soon outgrew the Servian walls, expanding to include areas on both sides of the Tiber River and up- and downstream, but it wasn't until around 270 AD that something had to be done about walls. Ten years earlier, due to outside pressure and internal fragmentation, the frontiers of the Empire had suddenly collapsed. Aurelian, probably a native of the Balkans, was one of two cavalry commanders under Emperor Gallienus. When Gallienus was assassinated in 268, the other cavalry commander, Claudius (II) became Emperor. Claudius lasted only 18 months and was succeeded by his brother Quintillus, who lasted only three months. Then Aurelian became Emperor and quickly began restoring Roman authority in Europe. He defeated invaders in central Europe, drove a tribe called the Juthungi from northern Italy and quelled a revolt over debasement of currency in Rome itself. He then won battles on the Danube, captured Septimia Zenobia in Palmyra (in the east of modern Syria) and co-opted a rival "Emperor", Tetricus, who controlled Gaul, Spain, and Britain, thus bringing the whole empire back under Roman control. Most importantly, he began the new walls. It took ten years to build walls about half of the current height, and by that time Aurelian was dead, assassinated by a group of his own officers who apparently thought they had been marked for execution in 275 AD.
The walls were completed by a successor, Probus, by about 280 AD. They were strongly but hastily built, incorporating so many existing structures (for example, the Pyramid of Cestius and the Amphitheatrum Castrense, near the south-eastern corner of the city) that they constituted about one sixth of its total length. Except where there were already built-in structures to serve as bastions, squared off towers were built every 100 Roman feet (about 30 meters) with nine interior splayed firing slits between towers. The towers had purpose-built windows to allow ballistae to chuck boulders at approaching enemies over an interlocking field of fire. Once again, however, the walls were not immediately needed -- the same folks that built them had pushed the battle zone away from Rome.
They were needed, however, early in the fourth century, and not against foreign invaders. Maxentius didn't have to contact a soothsayer to learn that his brother-in-law and archrival, Constantine, was on his way to Rome. Max quickly doubled the height of the Aurelian walls by adding a second, narrower, but still formidable level. He also massively re-fortified the major gates and walled off many minor ones. Then Maxentius made one of the worst military blunders of his millennium: he marched out of his impregnable city and engaged Constantine at Saxa Rubra, nine miles north of the Milvian Bridge. Constantine routed Maxentius and drove his army back toward the narrow bridge, and, in the melee around the bridge, Maxentius fell into the Tiber and drowned. Constantine marched into Rome unopposed, and the newly heightened walls were untouched.
Rome's walls did not protect the city from waves of barbarian invasions that followed in succeeding centuries, but not because of any problems with their design or structure. There simply weren't enough people left to man the gates, much less the walls, after Constantine started the mass exodus by moving the capital to Constantinople. The walls were kept in repair as well as they could be, and in the middle of the 9th century Pope Leo IV built walls on the Trastevere side of the Tiber that brought the Vatican within the circuit. Other Popes kept the city walls and gates in repair (especially in the circum-Vatican sectors) as their temporal power over the city grew. Urban VIII (Pope from 1623-44) extended the Vatican walls southward along the Janiculum ridge making much of the Aurelian wall along the east bank of the Tiber unnecessary, and that section quickly disappeared into other construction projects.
That's the way things stayed -- since Urban's extension, the "Aurelian/Leonine/Urbanine" walls have pretty much run along the same municipal boundary that Marcus Aurelius and Commodus laid out for taxation purposes at the end of the second century AD.
By the time the Bersagliari arrived outside Porta Pia on September 20, 1870, the formidable looking walls were no match for the weapons of that day. The bombardment that blew the massive hole in the wall lasted only a few hours, and it was over so quickly that the Bersaglieri who poured through had to slow-march down Via XX Settembre to allow the Pope and his court enough time to escape from the Quirinale Palace to the Vatican. Photographers were on the scene to record the breach in the wall. (There are still a few cannonballs stuck in the wall from the bombardment.) Since the completion of Italian unification, the Aurelian walls have had no real military significance.
Unlike some medieval European walls, only a small part of Rome's circuit, in the south, is walkable. Go to http://www.initaly.com/regions/classic/walls.htm for information.
You can walk along, if not in or on, almost all of the Aurelian walls, and there is a very good web site that gives maps, pictures, annotated routes, and accurate, detailed information: http://www.geocities.com/mp_pollett/walls.htm.
Lacus Curtius has two sites on the Pomerium: http://www.ukans.edu/history/index/europe/ancient_rome/E/Gazetteer/Places/Europe/Italy/Lazio/Roma/Rome/.Texts/PLATOP*/Pomerium.html and http://www.ukans.edu/history/index/europe/ancient_rome/E/Roman/Texts/secondary/SMIGRA*/Pomoerium.html.
Information on the Servian and Aurelian gates and surrounding sections of the walls is at http://www.ukans.edu/history/index/europe/ancient_rome/E/Gazetteer/Places/Europe/Italy/Lazio/Roma/Rome/.Texts/PLATOP*/portae.html.