Circus Maximus from the 4th century CE model (drawing) of Rome at EUR (photo courtesy of VRoma)

There were three enclosures for chariot racing in Juvenal's time in Rome, but the Circus Maximus (see 16th century reconstruction drawing ), seated in the valley between the Palatine and Aventine hills, was the oldest (reputedly built under the kings) and largest. It consisted of long parallel sides and one semi-circular side with tiered seating and an open end for the starting gates; down the center was an island (spina; see drawing of the obverse of a silver sestertius of Trajan) that cut the field into two tracks and held at a height lap markers (eggs and dophins) and the turning posts.
Additional images and information about the Circus and chariot racing may be found at VRoma's Circus Maximus.

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Marble chariot race reliefs from sarcophagoi of the 2nd and 3rd century CE (photos courtesy of VRoma)