from Roman Imperial Architecture
As so often in late antiquity the imperial residence was accompanied by a circus, which is estimated as having held about 15,000 spectators, and which offers an unusually detailed picture of one of the racecourses that played such a large part in the social, and frequently also the political, life of later antiquity. The architectural niceties were many. Here one can see, for example, the ingenious irregularities of plan which ensured a fair start for the competitors in the outer lines; the starting-gates (carceres) set between the traditional pair of flanking towers (oppida); the two turning-points (metae) at either end of the central barrier (spina), which was placed well off-axis so as to allow for the crowding of the initial lap; the imperial box, overlooking the finishing line, and a second box near the middle of the opposite side for the use of the judges and organizing officials; the entrances and exits for the ceremonial parades of the contestants; and on the central point of the spina, in imitation of the Augustan obelisk in the Circus Maximus, the site of the Egyptian obelisk which Maxentius brought from Domitian's Temple of Isis in the Campus Martius and which Innocent X in 1651 retransported to the city to adorn Bernini's fountain in the Piazza Navona [which was the Circus of Domitian, also in the Campus Martius]. Constructionally the building is of interest for its bold use of alternative courses of bricks and of small tufa blocks, and for the large hallow jars used to lighten the mass of the vaulting that carried the seating, both characteristically late features which are discussed in greater detail later in this chapter.
John Bryan Ward-Perkins, Roman Imperial Architecture (New York: Penguin Books, 1981), pp. 421-424.
REPORTAGE- Rhythm & Gender
Why do you think Piranesi first deliniated all the circuses of the first printing of the Ichnographia Campus Martius in a stylized manner, and then (unnoticed for over 200 years) changed all the circuses into copies of the Circus of Maxentius in the second printing of the Ichnographia Campus Martiis? Piranesi sure knew how to paint a quaestio abstrusa!