The Amphitheatrum Castrense: Most visitors to Rome visit the Colosseum but never find their way to Rome's second ancient amphitheater. Those of us who live here know all about the Amphiteatrum Castrense, of course. At various times during the life of ancient Rome, other amphitheaters also existed. (Vestiges also remain of the small training arena in the school for gladiators that was just to the east of the Colosseum, but that's another story).
Perhaps the most curious amphitheater ever built in Rome was an early wooden structure erected, fittingly, by a man named Gaius Scribonius Curio, in 59 BC. It was built when Curio was Pontifex Maximus (High Priest), it was the first amphitheater built in Rome, and it was really two theaters built back-to-back. They could be pivoted, with the audience inside, to make an oval amphitheater. Nothing of this remarkable construct has ever been found, but Cicero, who was a political enemy of Curio, wrote about it rather derisively. Statilius Taurus built Rome's first permanent amphitheater of stone in 29 AD, probably east of Via del Corso near the southern end of Piazza Santi Apostoli. It was destroyed in Nero's fire. Nero himself had also later built a wooden amphitheater in the Campus Martius, and it was probably west or north of the Pantheon, because it survived the fire. Nothing remains of either structure. It seems reasonable to imagine that in a city as large as Rome, several additional amphitheaters may have existed as well, although none are recorded in extant sources.
We aren't even absolutely sure that this amphitheater really was the structure named "Castrense" in ancient sources. It is, however, a major building in about the right place, matching a late antique catalog of buildings within the City. It is only by interpreting "Castrense" as meaning "for the Imperial court" that the identification has been made -- the area of Rome around the site included the imperial palace of the time and other structures belonging to the emperors and their toadies. The name, however, also might suggest a special military entertainment arena, since castrum normally means a military camp.
The undoubted reason for the survival of any remains of this particular arena was its incorporation into the Aurelian walls. In 271 AD, the Emperor Aurelian thought that the weakened Empire was vulnerable to attack by northern barbarians and decided to re-fortify Rome. Speed and economy required that any large pre-existing structures should be incorporated into the new walls. All the arches of the amphitheater that faced out -- about a third of the circumference -- were simply bricked up. The Pyramid of Cestius was incorporated in the walls at the same time. In fact, the barbarians did not invest Rome, and the city wasn't captured for another 40 years when Constantine took it from Maxentius. (Another 'nother story.)
But Constantine did take the city and, when his mother, Helena, brought the "True Cross" to Rome From Jerusalem, Constantine authorized the transformation of the Porticus next to the amphitheater into the basilica of San Croce in Gerusalemme. The amphitheater ultimately became the oval-shaped walled churchyard, and that further accounts for the fact that it still exists, much altered, today.
The amphitheater was a broad (88 meter and 75.80 meter) oval. It was built entirely of brick-faced concrete and originally had three storeys on the exterior -- drawings of the sixteenth century still show these even though the top storey and all but a very small part of the middle one were already gone by then. Measured drawings, especially two by Palladio drawn from earlier sources, provide us with most of our information.
The lowest storey (the only one remaining) is still embellished with engaged pillars with Corinthian capitals made of specially molded brick. There are no bases, but there are plinths of travertine blocks. In the second storey the engaged order was replaced by pilasters, also Corinthian. In the third storey rectangular windows replaced the arcades, and the elongated pilasters of the ornamental order, also Corinthian, carried only plinth-like sections of entablature as a crowning finish. Between these were footings for the masts that carried awnings for the spectators.
The cavea seems to have been very narrow, restricted to a single rank of nine rows of seats, but Palladio's drawing suggests there was additional seating above, in a summum maenianum. There were chambers beneath the arena floor, but there is no information about how these were arranged.
The footing of the lowest order of engaged columns stands high above ground level today, and it has been suggested that at the time of the building of the Aurelian walls the ground was considerably lowered. But the brick facing descends smoothly to the top of the concrete foundation, showing that the level has been lowered only a foot or two around most of the circumference. There can, therefore, have been only a limited access, probably mainly on the axes of the amphitheater.
After much debate, scholars have now reached a general agreement that the building should he dated to the time of Elagabalus (Emperor from 218-222 AD), who was responsible for extensive construction in this area. The complete absence of brick-stamps seems to support this date -- none were used at the time.
A fine overhead view of the area, centered on the church and the amphitheater is at http://cidc.library.cornell.edu/romephotos/photos/284.jpg
The description from The Platner and Ashby Topographical Dictionary is at http://www.ukans.edu/history/index/europe/ancient_rome/E/Gazetteer/Places/Europe/Italy/Lazio/Roma/Rome/.Texts/PLATOP*/Amphitheatrum_Castrense.html
The Lacus Curtius description is at http://www.ukans.edu/history/index/europe/ancient_rome/E/Gazetteer/Places/Europe/Italy/Lazio/Roma/Rome/walls/Aurelian/Amphitheatrum_Castrense/home.html
A short summary of the history of San Croce with a timeline of area construction is at http://web.tiscali.it/romaonlineguide/Pages/eng/rcristiana/sCH2y6.htm
A more detailed, but still manageable take on S. Croce is at http://home.online.no/~cnyborg/crocegerusalemme.html, where you can also find links to hundreds of other Rome churches.
P.S.: It is thought that the Curio rotating wooden theaters were the first amphitheater anywhere. Amphitheater simply means "theater on both sides" (Greek: ajmfiv=both sides, and qevatron=theater) and Curio's twin theaters obviously filled the bill. Amphitheaters in general are described in Smith's Dictionary at http://www.ukans.edu/history/index/europe/ancient_rome/E/Roman/Texts/secondary/SMIGRA*/Amphitheatrum.html