Short answers, pulled from the story.
A Roman circus was a large open-air venue used mainly for chariot races. It sometimes served other public purposes, and along with theatres and amphitheatres it was one of the principal entertainment venues of the ancient Roman Empire.
The Circus Maximus had a track length of approximately 621 metres and a width of approximately 150 metres. Its seating capacity is estimated at 150,000, with some estimates reaching as high as 250,000 spectators.
The spina was the median strip running along the centre of the Roman circus track. It typically featured ornate columns, statues, and commemorative obelisks, and marked the dividing barrier around which chariots raced anticlockwise.
The carceres were the starting gates of a Roman circus. They had a distinctive slanted and slightly curved plan designed to equalise the distances each competitor travelled before reaching the first straight section of the track.
The only known chariot-racing track in Britain was discovered at Colchester in 2004 by the Colchester Archaeological Trust. The track measured approximately 450 metres long and 75 metres wide, with evidence of eight starting gates.
As of a count dated to the 1st of April 2012, at least 63 Roman circuses had been identified. They are spread across sites from Britain and Spain in the west to Jordan, Syria, and Turkey in the east.