It might be useful to gather all the ancient testimonies on the Circus of Nero / Circus of Gaius, on the Vatican, and see what they do, and do not, tell us.
Pliny the Elder, Natural History, NH book 36, chapter 14 / section 70 (Loeb, vol. 10, p.54-5):
Divus Claudius aliquot per annos adservatam, qua C. Caesar inportaverat…
The ship used by the emperor Gaius for bringing a third [obelisk] was carefully preserved by Claudius of Revered Memory, …
Pliny the Elder, Natural History, NH 36, chapter 15 / section 74(Loeb, vol. 10, p.58-9.)
Tertius [obeliscus] est Romae in Vaticano Gai et Neronis principum circo — ex omnibus unus omnino fractus est in molitione …
The third obelisk in Rome stands in the Vatican circus that was built by the emperors Gaius and Nero. It was the only one of the three that was broken during removal.
This is rather loosely translated. Literally: “In the Vatican circus of the emperors Gaius and Nero”.
Pliny the Elder, NH, book 16, ch. 76, 201-2 (Loeb, vol. 4, p.518-9.):
abies admirationis praecipuae visa est in nave quae ex Aegypto Gai principis iussu obeliscum in Vaticano circo statutum quattuorque truncos lapidis eiusdem ad sustinendum eum adduxit ;
An especially wonderful fir [tree] was seen in the ship which brought from Egypt at the order of the emperor Gaius the obelisk erected in the Vatican Circus and four shafts of the same stone to serve as its base.
Suetonius, Caligula, 54. 1-2 (Loeb, vol. 1, p.486-7).
LIV. Sed et aliorum generum artes studiosissime et diversissimas exercuit. Thraex et auriga, idem cantor atque saltator, battuebat pugnatoriis armis, aurigabat exstructo plurifariam circo ;
LIV. Moreover he devoted himself with much enthusiasm to arts of other kinds and of great variety, appearing as a Thracian gladiator, as a charioteer, and even as a singer and dancer, fighting with the weapons of actual warfare, and driving in circuses built in various places;
Here there is an error in the Loeb translation: it ought to read in a circus built in various fashions.
Suetonius, Claudius, 21. (Loeb, vol. 2, p.40-41):
Circenses frequenter etiam in Vaticano commisit, nonnumquam interiecta per quinos missus venatione. Circo vera Maxima marmoreis carceribus auratisque metis, quae utraque et tofina ac lignea antea fuerant, exculto propria senatoribus constituit loca promiscue spectare solitis;
He often gave games in the Vatican Circus also, at times with a beast-baiting between every five races. But the Great Circus he adorned with barriers of marble and gilded goals, whereas before they had been of tufa and wood, and assigned special seats to the senators, who had been in the habit of viewing the games with the rest of the people.
Tacitus, Annals, book 14, 14 (Loeb, vol. 4, p.130-131):
XIV. Vetus illi cupido erat curriculo quadrigarum insistere, …. Concertare [e]quis regium et antiquis ducibus factitatum memora[ba]t, idque vatum laudibus celebre et deorum honori datum. … Nec iam sisti poterat, cum Senecae ac Burro visum, ne utraque pervinceret, alterum concedere, clausumque valle Vaticana spatium, in quo equos regeret, haud promisco spectaculo. Mox ultro vocari populus Romanus laudibusque extollere, ut est vulgus cupiens voluptatum et, se eodem princeps trahat, laetum.
14. It was an old desire of his [Nero’s] to drive a chariot and team of four, …. “Racing with horses,” he used to observe, “was a royal accomplishment, and had been practised by the commanders of antiquity: the sport had been celebrated in the praises of poets and devoted to the worship of Heaven. … He could no longer be checked, when Seneca and Burrus decided to concede one of his points rather than allow him to carry both; and an enclosure was made in the Vatican valley, where he could manoeuvre his horses without the spectacle being public. Before long, the Roman people received an invitation in form, and began to hymn his praises, as is the way of the crowd, hungry for amusements, and delighted if the sovereign draws in the same direction.
Cassius Dio, 59, 14:
[Caligula poisoned] … the horses and charioteers of the rival factions; for he was strongly attached to the party that wore the frog-green, which from this colour was called also the Party of the Leek. Hence even to‑day the place where he used to practise driving the chariots is called the Gaianum after him.
Chronography of 354, section XIV (CVRIOSVM VRBIS REGIONVM XIV CVM BREVIARIVS SVIS; and NOTITIA REGIONVM VRBIS XIV – both have same text here).
REGIO XIIII TRANSTIBERIM continet Gaianum et Frigianum
REGION 14, TRANS-TIBER contains: The Gaianum and the Phrygianum
The Platner and Ashby entry is worth including, for inscription material which I am unable to access:
Gaianum: an open space in Region XIV (Reg. Cat.; Hemerol. Filoc. ad V Kal. April., CIL I2 p314), south of the naumachia Vaticana and east of the via Triumphalis, where Caligula was fond of having horse races (Cass. Dio LIX.14). From inscriptions found in the vicinity (CIL VI.10052‑4, 10057‑8, 10067, 33937, 33953; BC 1902, 177‑185) it appears to have been surrounded by statues of successful charioteers (HJ 662; DAP 2.viii.355‑60; BC 1896, 248‑9).
What do we learn from this? I think we may reasonably state the following:
- Gaius – Caligula – practised chariot-racing, in an area known as the Gaianum, after his name.
- He had a circus which was constructed “plurifariam” – out of odds and ends.
- He also erected an obelisk in it.
- Claudius used the “Vatican circus” for shows, but, unlike the Circus Maximus, nothing says that he rebuilt it in marble.
- Nero used a chariot-racing area on the Vatican, initially as a private location, but then invited spectators, as Claudius had done.
- The circus was known as the Vatican circus of the emperors Gaius and Nero.
- Since it was named after Nero as well, at least in the time of Pliny, presumably Nero did substantial construction work there.
- An area on the Vatican was known as the Gaianum in the late 2nd century, and the name was still in use for the area in the 4th century.
Since we know that there was a circus, of “Gaius and Nero”, all these events most naturally relate to a single place, on the Vatican. The location of that place is defined into modern times by the position of the obelisk, which stood on the south side of Old St. Peter’s basilica and is now in the St Peter’s square of the new basilica.
The regionary catalogues also tell us that the circus was close to the Vatican temple of Cybele, the Vatican Phrygianum.