Short answers, pulled from the story.
Lucius Tarquinius Priscus built raised wooden perimeter seating for Rome's highest echelons during the Regal era. His grandson Tarquinius Superbus added the first seating for citizen-commoners on the Aventine side of the track.
Augustus rebuilt Ceres' temple above the starting gates after it was probably damaged in the fire of 31 BC. He brought an obelisk from Heliopolis and erected it midway along the dividing barrier of the Circus at that time.
Early Christians suffered damnatio ad bestias where condemned individuals were exposed to wild animals as capital punishment. Some accounts connect these executions involving animals with the reign of Nero following the Great Fire of Rome in 64 CE.
The original track now lies six meters beneath the modern surface due to accumulated debris and flooding. After the 6th century the stadium fell into disuse while lower levels were gradually buried under waterlogged alluvial soil.
Sun-god Apollo served as divine patron of the Circus with his sacred obelisk towering over the arena close to the finishing line. Castor and Pollux were patrons of horses horsemen and the equestrian order born from eggs symbolizing race-laps.