Ancient History & Civilisation

Chapter One

“The Size of the Plain Is Remarkable”: Defining the Limits of the Campus Martius in Time and Space


For many travelers to Rome today, the center of the city is often considered the area around the nineteenth-century monument to King Victor Emmanuel II, an imposing marble structure with a high colonnade that looks north over tightly spaced buildings and a busy traffic circle. Built hard up against the Capitoline Hill, a natural landscape feature known today as the Campidoglio, this memorial to the first king of a unified Italy hides from view the hill that once dominated the ancient skyline as seen from the northern reaches of the city. But wander off to the western side of the monument, and the visitor discovers the ramped carriage steps leading to the Campidoglio, the perfectly balanced space created by Michelangelo in the sixteenth century. From this high vantage point, the modern observer's gaze extends to the northern horizon across a plain dominated by tile-roofed structures from the seventeenth through nineteenth centuries. At the viewer's right is the very straight Via del Corso running north from the Piazza Venezia, but the undulating Tiber River on the left is now hidden by trees and a floodwall (Figure 1).

1. View north from the Capitoline Hill with modern Corso (ancient Via Lata/Via Flaminia) on the right. (Photo: Paul Jacobs)

Three millennia ago the Capitoline was a very different place. As a result of lava flows from volcanic eruptions over the previous 600,000 years, nearly 100,000 square meters of hilltop formed a rugged saddle of tufa resting between two higher peaks.1 A small village of wooden huts sat on the relatively flat space between the rises, with similar villages dotting Rome's other hilltops.2 Instead of an urban landscape lying to the north of the Capitoline, there was a marshy plain punctuated by woods and shallow pools of water. Caught between the Tiber River to the west and the Pincian and Quirinal Hills to the east, the flatland became a lake for many days during the seasonal inundations from the river's floodwaters. Further north, where the Tiber curves close to the hills, a small volcanic fissure hinted at the region's seismic instability. A narrow stream, later known as the Petronia Amnis, wended from the Quirinal through the marsh before emptying into the Tiber (Plan 1).3 Few, if any, artificial features stood between the trees and wetland pools on the plain to suggest human encroachment.4 This, however, was the area that – many centuries later – Romans would know as the Campus Martius, the Field of Mars.

According to ancient writers, the city of Rome was founded in 753 B.C.E., but there is little archaeological evidence to suggest that at that time it was much more than a cluster of humble villages.5 While a crude stone wall enveloped the Palatine Hill to the southwest of the Capitoline, and wood and thatch huts sat along the Palatine's crest, the most famous of the city's republican gathering places, the Roman Forum, was then a mere swampy lowland populated with a smattering of small dwellings and burial plots.6The northern plain would have shown little evidence of human intrusion, and although a century after the city's mythical founding other areas of Rome had changed dramatically, the floodplain to the north of the Capitoline was still open and marshy with few, if any, man-made structures.7 The villages resting on Rome's hills and tucked in its lower valleys had, by the sixth century B.C.E., expanded and converged into a city of stone, wood, and terracotta covering approximately 2.8 square kilometers. Fortified walls were under construction, and in the valley south of the Capitoline, the primitive wooden huts were cleared away to make space for the building of the Roman Forum.8 An open sewer channel, later covered and known as the Cloaca Maxima, helped to drain the basin of the Forum.9 On the lower of the two Capitoline peaks, the great Temple of Jupiter Optimus Maximus (Jupiter Best and Greatest) rose on a terrace approximately 3,300 square meters in area.10 With white stucco walls, timber beams, and a wooden roof crowned by a statue of Jupiter in a chariot,11 the enormous temple served as Rome's most significant religious structure. The open area that surrounded the temple functioned as the meeting place for Rome's first assembly, the Comitia Calata.12

With the pomerium located at the foot of the Capitoline, the open plain to the north began to be used for military musters. Roman men of fighting age assembled in the marshland every spring before heading off on a path that led north in the direction of hostile tribes in Etruria. Annual equestrian contests were also held in the field. With the possible exception of a small altar to Mars near the center of the field, it was not until two more centuries had passed that visible changes finally came to the marshy field north of the Capitoline.13 During the fifth century B.C.E., a large clearing was prepared about 300 meters beyond the hill in which citizens would congregate every five years to be counted in a census.14 Known as the Villa Publica, the gathering space remained free of permanent structures, although a portico and buildings were added two centuries later during a renovation (Plan 2, No. 4).15 Soon after space was cleared for the Villa Publica, a temple was erected on the southern edge of the field. Dedicated in 431 B.C.E. to Apollo Medicus (Apollo the Healer), the temple was raised in response to a plague that had recently ravaged the city (Plan 2, Inset C).16 The city continued to grow to the south and east with walls, temples, public buildings, aqueducts, and housing, but the northern plain was slower to see change. Roman writers do not report construction of a second significant structure in the plain before 296 B.C.E., when a temple to the war goddess Bellona was vowed and a few years later dedicated next to Apollo's temple (Plan 2, Inset C).17

Three decades later, the pace of construction quickened with the advent of the three Punic Wars (264 to 146 B.C.E.). The open space north of the city walls soon became a popular site for the placement of temples vowed by generals. A vegetable market, the Forum Holitorium, just to the northwest of the Capitoline and near to the temples of Apollo and Bellona, became the location for three temples (Plan 2, Inset D).18 By the Forum Holitorium and the temples of Apollo Medicus and Bellona, a site for temporarymarkets and public meetings developed. Over time, the space known as the Circus Flaminius would be articulated by temple precincts and portico complexes (Plan 2). For the first time, the center of the plain became a construction site as well. In and around a sacred zone now known as the Largo Argentina (Plan 2, Inset A), temples of rather obscure water deities were erected. At least sixteen temples, or more than half of Rome's temples vowed during the period of the Punic Wars, rose on the floodplain north of the city walls.19

Whereas the orientation of the Circus Flaminius and the temples on its edge generally followed the path of the Tiber River to its west, the republican temples in the Largo Argentina district were built along a north-south axis, a topographical pattern that was expanded in future centuries of ancient Roman construction. The earlier dirt road used by mustering troops was monumentalized and renamed the Via Flaminia (Plan 2). It became a major Roman highway jammed with soldiers heading off to distant northern and western battlefronts, as well as farmers and merchants transporting agricultural products, imported items, building materials, and domestic animals to and from the city center. The northern marshland also became a destination point for citizens participating in Rome's numerous religious festivals. Temporary stages and bleachers were erected near temples for theatrical events connected with temple dedications and annual celebrations. A practice track for horse racing likely was cleared near the Tiber. The area around the Circus Flaminius would also attract spectators to the gathering point for triumphal parades awarded by the Senate to successful military generals.

By the beginning of the first century B.C.E., clusters of temples and some porticoes were located in the southern and central portions of the plain, reflecting the captured wealth from Rome's successful foreign conquests. The perimeter of the Circus Flaminius was now clearly defined by permanent edifices, including two temples of Hercules (Plan 2, Nos. 7 and 10) and temples of Juno Regina (Juno the Queen) and her consort Jupiter Stator (Jupiter the Stayer), both enclosed, at least in part, by the Porticus Metelli (Plan 2, Inset B). A temple of Neptune (Plan 2, No. 11), one of Mars (Plan 3, No. 23), and a temple of the twin horse tamers Castor and Pollux (Plan 3, No. 19) also rested on the edge of the circus. A portico built by Gnaeus Octavius, an ancestor of Rome's first emperor Augustus, ran along the northeast side of the Circus Flaminius, either covering over or parallel to a major street leading into the heart of the northern field (Plan 3, No. 24).20 In the Largo Argentina region, a round temple now identified as honoring Fortuna Huiusce Diei (the goddess of the present day) was added (Plan 3, Inset A). The northern portion of the field that spread its way up to the narrow throat between the Tiber and the Pincian Hill remained open and relatively undeveloped. Because the plain was beyond thepomerium, burials were allowed there.

The late republic in Rome was a period of great men jockeying for power with the means to use architecture as a tool to promote their political ambitions. No longer needed for military musters, the flatland north of the Capitoline had sufficient space for enormous structures to reflect the glory of outsized personalities. Moreover, because of its associations with Rome's mythic beginnings, it provided an appropriate location to realize these projects. Temporary wooden entertainment venues became larger and more elaborate, and in the mid-first century C.E. Rome's first permanent theater rose just west of the Largo Argentina. Erected by one of the republic's most successful generals, Pompey the Great (106–48 B.C.E.), the enormous stone theater was complemented by a large open space fronted on three sides by an ornate portico (Plan 3, Nos. 25 and 26). Dedicated in 55 B.C.E., Pompey's theater was then the tallest structure on the field north of the Capitoline and dominated the landscape.21

Nearby, Roman citizens congregated in a precinct known as the Saepta for voting (Figure 2) in the centuriate assemblies (comitia centuriata) and tribunal assemblies (comitia tribunata). As an architectural counterpoint to the constructions of his rival Pompey,Julius Caesar (100–44 B.C.E.) conceived of a plan for replacing the Saepta with a massive colonnaded voting precinct, which was finally brought to fruition under the reign of Augustus (Plan 3, No. 7). While not leaving a permanent mark on the northern plain, Julius Caesar used it for grandiose displays, such as mock naval battles in an artificial basin called the Naumachia Caesaris. He even conceived a plan for straightening out the Tiber and adding land to the east bank.22 Caesar's proposal would have allowed an expanded plain to house Rome's growing population.23 Although the scheme was never implemented, the field was beginning to see private development, much of it unauthorized.24

2. Two citizens casting ballots. Reverse side of a denarius of P. Licinius Nerva (113–112 B.C.E.). (Photo: Trustees of the British Museum)

Besting Pompey for bragging rights in the field north of the Capitoline proved elusive for Julius Caesar. Ironically, Caesar defeated his rival at the Battle of Pharsalus in 48 B.C.E. yet was felled by assassins in an annex to Pompey's theater. With Caesar's death came further political turmoil that resulted in the rise of Octavian, Caesar's adopted son, who became Rome's first emperor, Augustus. Octavian was very familiar with the open field north of the city. In his youth, he went riding on the plain, and it was there that he camped his troops when he marched back to Rome in 43 B.C.E. to claim his rightful inheritance following Caesar's death.25 After defeating the forces of Mark Antony and Cleopatra VII at the Battle of Actium in 31 B.C.E., Augustus began a building program that saw an extraordinary transformation of the former marshland. Apart from addressing renovations to existing structures such as the Saepta, now the Saepta Julia, Augustus ordered the erection of new buildings, including his future burial site. A magnificent circular tumulus, the Mausoleum of Augustus dominated the northern horizon and reset the limits of the developed area to the north (Plan 3, No. 1).26

An undifferentiated area during the previous century, the northern part of the plain was highly organized during Augustus's reign with structures that related to one another, reflecting the substantial imprint of the emperor's building program. Along the Via Flaminia south of the Mausoleum, a square marble altar complex known as the Altar of Augustan Peace (Ara Pacis Augustae) was dedicated in 9 B.C.E. following Augustus's safe return from triumphal campaigns in Gaul and Iberia (Plan 3, No. 3). Adjacent to the altar complex was a horologium composed of an obelisk of red-gray Egyptian granite surmounted by a sphere, the shadow of which fell on a bronze marker inset in travertine (Plan 3, No. 2).

Augustus's chief military commander, Marcus Agrippa (63–12 B.C.E.) was given the task of transforming the center of the field. These projects included completion of the Saepta Julia, construction of a building to count votes (Diribitorium), Rome's first imperial bathhouse (Thermae Agrippae), an artificial lake (Stagnum), and the original Pantheon (Plan 3, Nos. 10, 12, 13, and 30). In order to supply the baths and other structures in the Campus Martius with fresh water as well as supply drinking water to villas across the Tiber, Agrippa ordered the construction of a new aqueduct, the Aqua Virgo. Oriented north-south and east-west, the Augustan era additions to the Field of Mars defined the organization of construction on the plain for several centuries thereafter. Placed in a depression that has been associated with the Goat Marsh (Caprae Palus), the site of Romulus's legendary ascension to the heavens, the Agrippan projects benefited from a subterranean drainage infrastructure that made the area now suitable for large buildings.27

Although Pompey's theater was the first permanent entertainment venue in Rome, others were soon raised on the northern field. An amphitheater used for gladiatorial games was constructed in an unknown location and dedicated in 29 B.C.E. by Statilius Taurus,the commander of Augustus's land forces at the Battle of Actium. To the southeast of the temples in the Largo Argentina, the noble L. Cornelius Balbus, who served under Julius Caesar, built a second stone theater complex (Plan 3, No. 16). Augustus had yet a third permanent theater placed close to the temples of Apollo and Bellona, naming it for his deceased nephew Marcellus (Plan 3, No. 17). The three theaters alone created permanent seating for as many as 45,000 Romans eager to attend theatrical performances and witness wild animal hunts and gladiatorial combat. In a few short decades, the plain had been transformed from a mostly open, marshy area with temples and wooden bleachers for occasional festivals to a daily draw for citizens for bathing, shopping, and entertainment.

By the death of Augustus in 14 C.E., many of the republican temples and other public structures located around the Circus Flaminius had been refurbished and new ones added, all dedicated to the greater glory of Augustus and the imperial family. The Porticus Metelli was reconstructed, expanded, and renamed the Porticus Octaviae after the emperor's sister (Plan 3, Inset B). The Temple of Hercules Musarum (Hercules of the Muses) was surrounded now by a portico (Porticus Philippi) erected by L. Marcius Philippus, Augustus's stepfather (Plan 3, Inset B). Several restored temples, including temples honoring the gods Apollo Medicus (now referred to as Apollo Sosianus), Jupiter Stator, Juno Regina, Neptune, Mars, and Felicitas, were rededicated on Augustus's official birth date, September 23.28

Although the Villa Publica in the earlier centuries had extended over approximately 150,000 square meters of the northern plain, its area had been pared down considerably by this time. The Theater of Balbus, the Saepta Julia, and the Diribitorium now filled the once open space where soldiers formerly gathered and the census had been taken.29 Colonnades enclosed gardens and, in the central portion of the plain, former marshland was being converted to tame public parks interspersed among monuments. The open fields – once vast – were rapidly shrinking.30 Nevertheless, sufficient open parkland in the northern and western reaches must have remained because, according to Strabo, space was available to accommodate chariot racing and equestrian exercises “without interference” and there was ground “covered with grass throughout the year.”

Despite the greater use of stone and concrete in the buildings in the northern plain, wooden structural timbers were employed throughout the area. This meant that fire was a constant danger in the Campus Martius, as elsewhere, in a city of about one million inhabitants. Devastating conflagrations periodically erupted there in the first century C.E., resulting in the need for reconstruction of many of the Augustan architectural monuments and providing space for new imperial projects. The Theater of Pompey was damaged by flames and repaired in 21 C.E., and the Amphitheater of Statilius Taurus was destroyed in the fire of 64 during the reign of Nero (r. 54–68 C.E.), and there is no indication that it was thereafter rebuilt. The great fire of 64 inflicted even more damage within the city walls, and the emperor temporarily used the area north of the Capitoline to house a population displaced by the flames.31 Just a few years previously, Nero had added a second great bathhouse, complete with gymnasium, the Thermae Neronianae,just northwest of Agrippa's baths (Plan 4, No. 18).32

A major fire in the spring of 80 C.E. was particularly destructive to the buildings north of the Capitoline.33 For three days and nights, flames raced through the plain. Smoke and fire poured out of the Saepta, the Baths of Agrippa, the Pantheon, the Theater of Balbus, the stage building at Pompey's theater, and many more structures.34 The Diribitorium – headquarters for firefighters in a previous blaze – was also a victim as its giant roof of larch wood came crashing to the floor.35 Next to the Saepta, temples less than a century old and dedicated to the Egyptian deities Isis and Serapis also burned (Plan 3, No. 5).36 While the emperor Titus (r. 79–81 C.E.) was in Campania attending to the disaster that befell Pompeii and Herculaneum a few months earlier, fire brigades raced to douse the flames whose sparks had even reached the Temple of Jupiter on the Capitoline.37

Many of the structures that were charred and in a state of collapse from the fire of 80 C.E. were restored by order of the emperor Domitian (r. 81–96 C.E.). The theaters of Pompey and Balbus, the Saepta, Agrippa's baths, temples of Egyptian gods Isis and Serapis, and the Pantheon were all substantially repaired and restored. The giant horologium of Augustus was relaid to improve its accuracy.38 Apart from reconstruction projects, Domitian left his own mark on the urban development of the plain. He reduced the Villa Publica to a small park one-tenth its previous size and erected in its center a pair of temples dedicated to the divine incarnations of his brother Titus and his father Vespasian, renaming the set of buildings the Divorum (Plan 4, No. 9). Next to that complex, the emperor commissioned the Temple of Minerva Chalcidica (Plan 4, No. 8).39 Just to the north of the Theater of Pompey, a smaller covered theater, the Odeum, was built (Plan 4, No. 20), as was a new venue for athletic competition known as the Stadium of Domitian(Plan 4, No. 19).

The relative wealth and political stability of the second century C.E. encouraged another spurt of construction with the addition of several buildings in the center of the once open field by Hadrian and his Antonine successors. Totally reconstructed under Hadrian, the Pantheon (Plan 4, No. 14) took the form we recognize today (Plate I). Hadrian also erected nearby a temple to his deceased mother-in-law Matidia (Plan 4, No. 13), and just east of the Temple to the Divine Matidia, Hadrian's successor Antoninus Pius(r. 138–61 C.E.) caused to be erected the Temple of the Divine Hadrian (Plan 4, No. 4). To the north a freestanding column with a base displaying a personification of the Campus Martius was erected in memory of Antoninus Pius and his wife Faustina by their adopted sons Marcus Aurelius and Lucius Verus. Near to this memorial, another tall marble column depicting battle scenes and a temple were later built to honor Marcus Aurelius (r. 161–80 C.E.).

Plate I Pantheon exterior and obelisk in the Piazza della Rotonda

During the reign of Alexander Severus (r. 222–35 C.E.), the Porticus Octaviae was burned and repaired, while the Baths of Nero was expanded and renamed for the emperor as the Thermae Alexandrianae (Plan 4, No. 18). The large open spaces of the Campus Martius had now vanished, and the construction of major public buildings on the once marshy plain was nearly over. The area north of the Capitoline had been filled with temples, baths, and theaters laced together with kilometers of porticoes enclosing beautiful gardens.40 Between the imperial structures were numerous shops, warehouses, and apartments. After 800 years, the Campus Martius was finally built out. By the mid-third century C.E. there was little, if any, open space left unencumbered. The massive Aurelian Wall defined the northernmost reach of the plain by the end of that century, encompassing within its nineteen-kilometer perimeter the area that is the focus of this study as well as much of the rest of the city (Plan 5). The pomerium had moved north as well, reaching at least as far as the center of the plain and possibly along the Aurelian Wall.41

Had Strabo been alive four centuries after the Augustan era, he would have likely marveled at the increased number of marbled colonnades, sacred precincts, theaters, and very costly temples against the stage backdrop of the “crowns of those hills that are above the river and extend as far as its bed.” The amphitheater he had observed was gone, but the Odeum and Stadium built after his day were active entertainment sites. What was no longer present in the fourth century C.E. to have caught a viewer's eye was the open space he had so admired, used “without interference” for “chariot-races and every other equestrian exercise,” “ball-playing” and “hoop-trundling,” and “the ground…covered with grass throughout the year.” The Campus Martius was now finely woven within Rome's urban fabric.


So where exactly in this plain was the area that the Romans called the “Campus Martius”? This question has been the subject of modern scholarly contention for generations. Roman writers do not inform the controversy because they used the term somewhat loosely in their descriptions. “Campus” generally referred to land lying outside of the pomerium and was sometimes, but not always, conjoined with the name of the nearest gate or associated with a particular structure or entity.42 Occasionally, early sources employed a broadly descriptive phrase for the location of a building while describing another structure or event as located “in campo,” leading some scholars to draw boundary lines around the Campus Martius on the basis of such distinctions.43 The use of the war god's name in connection with “campus,” at least in extant literary sources, was a mid-first-century B.C.E. development and even after that point not consistently employed. Firm conclusions about its use as a distinguishing spatial marker are, therefore, difficult to reach.

Gazing down at the plain from any of the surrounding hills, an ancient Roman could observe a few natural reference points – the Tiber on the west and northwest, the row of hills on the east and at least two drainage channels that flowed from the hills to the river.44 Until the addition of man-made structures, there was nothing tangible to separate one part of the floodplain from another. Ancient writers tell us of several other discernible areas of the early Field of Mars: an oak grove somewhere north of the Tiber Island,the Caprae Palus in the center of the space, and fields of grain.45 In the fifth century B.C.E., there were finally two structural imprints defining the space, the Villa Publica and the Temple of Apollo Medicus. It is generally thought that the path north abutting the Villa Publica and that later became the Via Flaminia served as the eastern edge of the Campus Martius (Plan 3).46

It has been argued that one of the two streams crossing the plain, the Petronia Amnis, formed a southern border to the Campus Martius, thereby excluding from the campus zone the large open area of the Circus Flaminus and the many temples that surrounded it.47 Support for excluding the structures in the Circus Flaminius from the Campus Martius is found in the fact that some ancient references describe their location as “in circo Flaminio” and not “in campo Martio.”48 Counterarguments are offered that, although the Petronia Amnis may have been a northwestern boundary to the Circus Flaminius, that fact does not necessarily mean it was the border of the Campus Martius, because the Field of Mars was likely subdivided into smaller tracts whose titles identified more precise locations.49 Moreover, some structures were noted as residing both “in circo Flaminio” and “in campo Martio.”50 Certainly, by his reference to “three theatres and an amphitheatre,” Strabo clearly believed that the Theater of Marcellus at the southern edge of the Circus Flaminius was within the same campus as the theaters of Pompey and Balbus northeast of the Petronia Amnis.51

With respect to the western line of the Campus Martius, there is no debate that the campus did not extend further to the west than the east bank of the Tiber River. It has been suggested, however, that it stopped short of the river's edge, following the line of an ancient southeast-northwest road running from the Circus Flaminius to a bridge (the Pons Neronianus) crossing the river about 1.3 kilometers upstream.52 The northern reach of the Campus Martius had no clear natural divider, but the bend in the river by the modern Ponte Cavour created a narrow throat of land between the Tiber and the Pincian Hill, extending just past the modern Piazza del Popolo. When it was constructed in the late first century B.C.E., Augustus's Mausoleum was the only major structure in this area. Located halfway between the river and the Via Flaminia, it likely created a strong visual signpost for the northernmost limits of the Field of Mars (Plan 3, No. 1). As with the southern border, however, this conclusion, too, is in dispute.53 The third-centuryC.E. Aurelian Wall,built about a half kilometer north of the Mausoleum, would have also provided a convenient demarcation for the northern edge of the Campus Martius, but it is not clear that it served that function (Plan 5).54

Although it is not possible to reconcile all of the disputes over the borders of the Campus Martius, part of the debate may be due to shifting definitions of the space by ancient viewers. As natural features became obscured by man-made structures, alternate lines of demarcation likely developed, making easier the exclusion of some portions of the plain while confusing the division from others.55 (Compare Plans 1 and 2.) Furthermore, the Campus Martius may have referred to different topographical conditions of the plain depending on context. Occasionally, the reference could be to the remaining open space as opposed to the entire built-up area.56 Recalling Rome's panorama from his exile on the Black Sea, Ovid wrote:

From my home, I turn my steps once more towards the beautiful city's regions and my mind surveys all those places using its own eyes. Now the fora, now the temples, now the theatres clad in marble, now the colonnades and their level grounds rise up before me, and now the grassy Campus Martius with its view of fair gardens, and the pools and canals and the water of the Virgo.57

There were some defining structures that separated the developed areas from the shrinking open spaces. In the late first century B.C.E., the arches of the Aqua Virgo that crossed the Via Flaminia and the north side of the Saepta Julia would have created a visual boundary line between the northern fields and the increasingly crowded southern campus.58 About a century and a half later, that boundary had pushed north about 152 meters to the road that connected the Pons Neronianus to the Via Flaminia. The closely sited Stadium of Domitian, Baths of Nero, Pantheon forecourt, Temple of the Divine Matidia, and Temple of the Divine Hadrian dominated the southern side of that cross street whose ancient name remains uncertain.59 (See Plan 4.) Their crowded placement must have made the fields to the north appear even more isolated.60

In 7 B.C.E., administrative order was imposed on the topography of the city, including the Campus Martius. In that year, Augustus reconfigured Rome's four main districts and added territory outside of the pomerium to create fourteen administrative regions. Composed of smaller wards known as vici, the regions were indicated by small stone markers, or cippi. Municipal services such as street maintenance and fire fighting were organized on a regional basis.61 All of the area west of the Via Flaminia to the river's edge and from the Capitoline to at least as far north as the Mausoleum of Augustus fell within Region IX.62 This would have included the monuments around the Circus Flaminius and the Theater of Marcellus and arguably the Forum Holitorium. Augustus's urban reorganization identified the Via Flaminia as the eastern boundary of that region. The land north of the Capitoline but east of the Via Flaminia extending to the Quirinal and Pincian Hills was relegated to Region VII.63 The fourth-century C.E. regionary catalogs,which provide names to the various regions, do not call Region IX the Campus Martius, however; instead, the tract is called “Circus Flaminius.”64 While at one time the pomerium could be used as a dividing line to separate the Field of Mars from the city, that designation, too, was subject to revision with cippi.65

This study uses “Campus Martius” to identify the plain north of the city from the Via Flaminia to the Tiber's edge and from the temples of Apollo and Bellona and the Forum Holitorium to the Mausoleum of Augustus, essentially the space encompassed by Region IX. Although there were apparently some distinct areas within the large plain, the whole seems to have been viewed as more than the sum of its parts. Cicero noted, for instance, “I think, indeed, that if the Campus Martius were to be divided, and if every one of you had two feet of standing ground allotted to him in it, still you would prefer to enjoy the whole of it together, than for each individual to have a small portion for his own private property.”66 The fact that the space had certain physical attributes and was beyond the pomerium until at least the mid-first century C.E. more likely influenced its architectural development than any topographical labels used in Augustus's regiones system.

Even when viewed under the most expansive definition, the Campus Martius was no larger than many parks in the modern world's great cities, and in some cases much smaller. Measured with the Via Flaminia as the eastern edge and including the Circus Flaminius area, the Field of Mars was only about 1.7 square kilometers.67 That is approximately half the size of New York's Central Park, or about two-thirds of the combined area of London's Hyde Park and Kensington Gardens. Paris's Bois de Boulogne is about five times larger. What the Campus Martius lacked in buildable area, it made up for in political, martial, and architectural significance.

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