21. A Second Look at the Colosseum: The Structure Itself

This podcast discusses the Colosseum as a building, including its overall shape, the huge awning that protected the spectators from the hot Roman sun, the foundations needed to support it, and its decorative elements. Our next episode will take up the kind of events that were staged in it.

In Episode Five we divided Ancient Rome into three, a Kingship founded by Romulus, a Republic founded by Lucius Junius Brutus, and an Empire founded by Julius Caesar’s adopted son Octavian, who gave himself the honorific title, Augustus. The Colosseum was inaugurated in 80 or possibly 81 AD, about 100 years into the third period of Ancient Rome, the period when Rome was ruled by emperors. It is thus a suitable example of the simple but crucially important point that most of what is left of Ancient Rome was built during the Empire. If we remember Rome primarily by its visible remains, we will be led to undervalue the Republic and forget the monarchy entirely, for their structures were simpler and have mostly disappeared.

Augustus, the first Emperor, boasted that he found Rome a city of bricks and left it a city of marble, which was an exaggeration, but an exaggeration of a truth. Rome under Augustus and his successors had more beautiful buildings than Rome under the Republic or monarchy.

The more difficult question is how to rank beautiful buildings as compared to other important political goods. If Rome’s most impressive architecture and art coincided with the period in which her citizens lost their liberty and became more morally degraded, the achievement would lose some of its luster. Because this seems like a good question to keep alive, I regret the use of the Colosseum as the icon of Ancient Rome. It expresses well the power, beauty, ingenuity, and efficiency of the Roman Empire, but the Colosseum was built for subjects, not for self-governing citizens. One reason Shakespeare’s Julius Caesar is such a gripping play is that it shows well that imperial power is compatible with moral decline, and may even require it.

Not all went well during the first century of the Empire, when Romans suffered under such depraved or incompetent successors of Augustus as Caligula, Claudius, and Nero. After a year of civil war in 69, Vespasian won control of the imperial purple and then was succeeded by his sons Titus and Domitian. Together they were the Flavian Dynasty, born of a civil war and ending when the third in their line, Domitian, returned to some of Nero’s tyrannical habits and got himself assassinated. Since Vespasian began building the Colosseum, and his sons Titus and Domitian added to it, the Romans called it the Flavian Amphitheater.

The Flavians located it in the drained bed of a lake that had been part of Nero’s outrageously large and lavish Golden House. (Click HERE for a short video.) Their political goal was to show the People of Rome that they would use Rome’s wealth for the public good, or at least for the public’s amusement, rather than for their own private consumption. Since the Flavians were the first to come to power by military success and without belonging to a distinguished family, it was especially useful for them to add to their authority by conspicuous public works. As the Roman poet Martial put it when the Colosseum was inaugurated, “Rome has been restored to herself, and what were formerly the delights of the master, [Nero], are now, thanks to you [Titus], the delights of the people.” True enough, but could Martial’s flattery of Titus also have meant also that the people’s delights were now just as depraved as those of Nero?

Several of the other major building projects were undertaken by the Flavians to help distinguish themselves from Nero. These include the Arch of Titus, the Baths of Titus, the Temple of Peace, the Temple of Vespasian and Titus, and the Stadium of Domitian, whose outlines are preserved in the shape of Piazza Navona.

Another reminder of the excess that was one of Nero’s trademarks is the name “Colosseum,” which was taken from a colossal bronze statue of a nude Nero, almost as tall as the Statue of Liberty. In a remarkable feat of Roman engineering, Hadrian found a way to move this enormous statue so it stood between the Temple of Venus and Roma, which he had just built, and the Flavian Amphitheater, which later took its familiar nickname from the statue. Of course the Flavians modified the statue so that it no longer represented the deeply loathed Nero, but a god, the god of the Sun, Sol. The Emperor Commodus later had it modified again so that it resembled him.

As a physical structure, what strike me most about the Colosseum are its general shape, its use of arches and vaults, its decoration, its size and basic materials, its foundations, and its velarium, that is, the vast awning that could keep many of the spectators in the shade.

In form, the Colosseum is an amphitheater, which means a double theater. The theater came first, and roughly speaking, the amphitheater is two very large theaters built so they face each other. Joining and enlarging the semicircular stages of each theater then results in a large oval arena surrounded by tiered seating.

The Colosseum was the largest Roman amphitheater, but it was not the first. It was preceded in Rome by several temporary structures and even by a more substantial amphitheater that burnt down in the Great Fire of 64, Nero’s fire. Pompeii had a permanent amphitheater a century before the Colosseum. Capua had one even before that.

Although they varied in size, all of the amphitheaters in the Roman Empire were ovals and were similar in general design. Most had a longer axis that is between 1.2 and 1.3 times the length of the shorter axis, so their shapes were similar, and yet the experts continue to debate the Colosseum’s precise shape. Was it an ellipse in the mathematical sense, or a multi-centric oval? I can’t contribute anything useful on this technical question, so I’ll leave it at calling the Colosseum an oval in the least technical sense of the term.  But here’s a more basic question: Why were all Roman amphitheaters ovals rather than circles? I have not discovered an authoritative answer, so I propose we begin thinking about it together.

Ovals would be more complicated to build than simple circles, for the curvature changes as we move along the perimeter. Once the form of the arena is determined, this exact but varying curvature must then be replicated in every row of seats and in the series of annular walls that encircle the arena, starting with the innermost wall around the arena and ending with the exterior wall. Since the Colosseum was built by workers working independently in four different quadrants of the project at the same time, which was necessary to finish the project in the relatively short period of ten years, I marvel at the challenge of achieving precise alignment on so many different points for so large a structure. This would be hard enough with a circular amphitheater, so why add the further complication of building an oval?

Because football and soccer fields are rectangles with a longer and a shorter dimension, it makes sense to put them into oval amphitheaters, not circular ones. If some gladiatorial events required a long straight space, as perhaps for a horse race or a hunt from horseback, this would answer our question, but I have never read that this was the case. Elongating a circle into an oval would increase the seating capacity without needing to build even higher. It would also increase the size of the arena in the center. Perhaps these considerations help explain the choice, but couldn’t similar results be achieved by increasing the diameter of a circular structure? An advantage of an oval that cannot be achieved by a circular amphitheater is the addition of a second kind of hierarchy into the seating arrangements. With both circles and ovals, the lower seats offer a better view of the action than the upper ones, so one way to show who is important is to sit them closer to courtside, so to speak. An oval adds a further advantage and distinction to those in the seats closer to the shorter axis, for they are closer to the center of the oval. This is why Jack Nicholson watched the Lakers from midcourt, not end-court. Thus, the oval offers an added way of sorting spectators into a hierarchy. And in fact the Emperor, the Vestal Virgins, and the other dignitaries sat on the shorter axis. Finally, a longer axis also allows for a longer parade to precede the games, just as the long nave of St. Peter’s—a departure from the original plan of Bramante and Michelangelo—allows for longer processions to and from the high altar. These are my best thoughts on the choice of an oval shape for Roman amphitheaters; I’d enjoy hearing yours.

We noted a few episodes ago that the main visual feature of the Colosseum and the Theater of Marcellus is a series of arcades—rows of arches—one on top of another. I can’t think of an Ancient site in Rome in which arches are supported merely by columns, though there are beautiful examples of such arcades in Roman churches, such as in the interior of Santa Sabina and the exterior of Santo Stefano Rotondo.

Since the arches of the Colosseum and of the Theater of Marcellus had to support the heavy load of a large wall above them, they were supported by thick and wide piers, which are like columns on steroids. But each thick pier is then decorated with a half-column facing outwards, which makes the pier seem a little less bulky. Like the arched openings themselves, they lighten the appearance of the outer walls. As was traditional, these simulated columns were decorated with either Doric, Ionic, or Corinthian capitals.

The eighty arched openings through the perimeter wall allow rapid entrance and egress, toward and away from the center of the Colosseum. Seventy-six entrances were numbered and were for the general public. The two on the short axis were for the Emperor and dignitaries, and the two on the long axis were for personnel involved in the games. The area under the seats of the Colosseum is divided between perpendicular walkways: annular walkways go around the structure, while radial walkways begin at the arches on the outside and aim at the center. Supporting the seats above the walkways are barrel vaults, which is the kind of vault formed when you take one arch and give it depth by, in effect, adding more arches just behind it. The result is like a tunnel, though often a very large and airy tunnel. Of course as the seats above slope downwards toward the center of the arena, the height of the corridors diminishes.

Things get a little tricky where the annular and radial barrel vaults cross, and I believe it was first in the Colosseum that the Romans devised what are now called groin or ribbed vaults to meet this challenge. Such vaults later became a common feature of brick, stone, and cement construction, and we will see them also in various of Rome’s churches. Try to imagine what must happen when two arched walkways meet at right angles, and then take a peek at the Show Notes for this podcast on my website. Better yet, go to the second floor of the Colosseum and look up.

The Colosseum has fared much better than other ancient structures, but much of it is missing. Half of the outermost wall is gone. All of its lavish decoration is gone. The seats are gone.  The substructure and animal pens are gone. The vast awning that could keep many of the spectators in the shade is long gone. Some features that may look at first like relics from the distant past turn out to be repairs carried out in later centuries. What is true of the Colosseum is also the case with other aspects of ancient Roman culture: some of the writings and political and moral ideas of the Ancient Romans are still available for study, but much has disappeared. Our logo of the Colosseum with a big notch in it can serve as a general reminder that much of Rome has vanished even if there is also much from which to learn.

The combination of attractiveness with functionality is one of the Colosseum’s hallmarks, but we should also stress that it is big, very big. It is the tallest building the Romans built, and just its outer wall required about 240,000 loads of stone carted in from quarries twenty miles away, partly on roads specially built for the purpose. I don’t know how to estimate the amount of cement needed, but it did not appear spontaneously on the site. Kilns had to be built to heat limestone to produce the lime required for the cement, and river sand or, better, pozzolana had to be found and dug out of the ground. The large travertine blocks were held in place especially by gravity but also by iron clamps. Scholars estimate that there were 300 tons of such clamps. I read that the project kept 20,000 or 30,000 workers busy, and I can believe it. To reduce the confusion on the building site and reduce the transportation burden, the stones were dressed at the quarry.

The Romans attempted to make the Colosseum attractive though decoration, of which there was quite a lot. The best-known contemporary scholar of Rome, Mary Beard, even refers to “decorative excess.” There were probably statues in each of the 160 arches on the second and third levels of the outermost wall, and there were forty large shields attached to the solid top level alternating with the forty windows. There were also paintings, stuccos, and literally tons of marble used as facings. Some of the stuccos stayed well enough intact to have been copied and imitated during the Renaissance.

It would be a gross exaggeration to say that all we can see of the Colosseum is like the visible tip of the iceberg, but much of the work the Romans did is underground. Consider first the structure’s foundations. The Colosseum was built on the site of a former lake, not over bedrock; and the building itself is tremendously heavy. In my neighborhood, even small wood-frame houses suffer foundation problems, so how did the Colosseum face this challenge?

The weight of the Colosseum is not distributed evenly over its entire oval footprint. In the center was the arena, which did not have to support anything besides gladiators and an occasional elephant, lightweights compared to the colossal structure itself. The heaviest weight was concentrated along the outer perimeter, where the walls are highest, thickest, and made of the most dense materials. The outer wall is approximately 150 feet high and has an average thickness of about 7.5 feet. Each linear foot of wall thus has 1,125 cubic feet of travertine stone above and behind it. There’s no risk of the weight of the wall crushing the travertine itself, but whatever is underneath it has got to be able to support a considerable load. As we move in from the perimeter, the annular walls become lower and thus have less mass pressing downward.

The experts disagree on some of the details of how the Romans kept the Colosseum from sinking, but part of the solution was to distribute the most extreme weight over a broader area, which is what we do when we wear snow shoes. In the case of the Colosseum, the idea was to build a huge ovoidal donut out of dense stone and bind it together with high quality Roman cement. Make the donut wider and longer than the Colosseum itself, so its large footprint would distribute the structure’s weight—and especially the weight of the tall outer wall—over a broader area. The donut could have a hole in the center because there was little to support in the center. If the concrete donut holds together, even if it settles a little, the integrity of the structure above should remain sound.

This may not have been a perfect solution, for we have seen that the southwest side of the Colosseum has lost its outer ring-wall. But before racing to blame the basic design of the foundation, I’d want to learn more about the fires, earthquakes, and abuse that the Colosseum has had to suffer over the centuries.

Prior to getting started on building the donut, the Romans had to drain the area, as it had been used as a lake in Nero’s absurdly grand “Golden House.” They thus built underground drains that carried water off in the direction of the Circus Maximus, in route to the Tiber. But water was also a recurrent problem for several different reasons. First, rainwater flowed toward the Colosseum, so drains were needed to carry it away. Second, the water table is high in this area, so there is always water in underground channels. To convince yourself of this, visit the beautiful church of San Clemente, just southeast of the Colosseum, and pay a few Euro to descend to the excavated lowest level of this church. Among other discoveries, you can learn that there really is underground water to contend with in this area, lots of it. Third, most scholars think that there were sea battles fought in the Colosseum in its first decade, before Domitian built the animal pens and storage areas under the arena floor. For these, water had to be brought in and later drained. Finally, the Colosseum was equipped with drinking fountains and latrines, so drains were again in order.

The result was a complex and robust system of drains. Some ran along the long axis of the arena and allowed water to flow in a channel under the structure. These are 4 feet by 11.5 feet, and have been explored by a scuba diving archeologist named Cristiano Ranieri. There are also annular channels that run around it, from which the water is then carried off toward the Circus Maximus and the Tiber.

Other tunnels through the foundation allowed for unseen entrances to the Colosseum by workers and gladiators alike. One such tunnel even allowed for the entrance of the Emperor. Called the “Passage of Commodus,” it was built by that crazy emperor, who actually fought as a gladiator himself. For reasons both fair and foul, he always escaped these battles with his life, though this did not keep him from being assassinated in the end.

It was surely not as easy to make the Colosseum’s ovoidal donut as it is to describe it. It was 36 feet from top to bottom and extended 18 feet beyond the perimeter of the outer wall. It thus required a tremendous quantity cement and dense stone. And to help bind the whole together, the Romans did not merely pour concrete; they hammered it into place. But they did not have to dig a massive hole into which to put this foundation. By choosing to situate the Colosseum in a low lake bed, the daunting tasks of digging dirt and hauling it away were partially replaced by the challenge of draining water.

The velarium, based on the Latin word for sail, was another impressive feature of the Colosseum. It was a huge awning that was manipulated by sailors, and it provided relief from the hot summer sun. No technical descriptions of it remain, so scholars are still busy pondering the likely configuration of such an awning, and how it might best have been controlled by ropes. The sailors must have been able to be “reef” the sails in a jiffy, if a storm blew in, and they presumably could also adjust them to suit the changing position of the sun.

Some scholars actually got to test various theories by experimenting with awnings on a bullring in Barcarrota, Spain, though questions remain. The difficulty of the undertaking should be clear to anyone who has gone sailing in even moderate winds. Consider also that one of these scholars estimated the total weight of the awning at 1 pound per square yard, which makes for a total of around 24 tons. This was no ordinary sail!

As for remaining physical evidence of the velarium, it is most clear in the uppermost tier in the outer wall. There are three ledge-like projections out from each of the eighty sections of this wall. Called corbels, these ledges supported the base of a wooden mast, which then passed through a hole in the projecting cornice, just above the corbel. These 240 masts would not have supported the awning directly; ropes suspended from them did the job. There used to be bollards down on ground level to anchor the rope, and it looks like a few of these still remain on the southeast side of the Colosseum. Mary Beard and other scholars say this can’t be correct, for their studies have shown that not securely planted in the earth that surrounds them.

Now that we are armed with a general understanding of the building itself, it is almost time to take up the various activities the Ancient Romans staged in their giant amphitheater. These will testify less to their prowess as builders than to their taste for violence.