Raised at the end of the 1st century, the amphitheatre is gorgeous. When Romans ruled the area, gladiators used to fight here. Its decoration was beautiful, with low relief on first floor's arches representing the circus games, a she-wolf breastfeeding Romulus and Remus, representations of god Pan (in fact, a phallus with goat legs!)...
But when Visigoths invaded the area in the 5th century, the circus game were banned. Barbarians stayed in Nîmes since 508, but Franks led by Clovis drove them out. All? Of course not! Some Barbarians stayed in the amphitheatre and surrounded it by huge ditches in order to transform it into a fortress, called castrum arenarum.
They raised houses inside in order to accommodate soldiers. On the Western door, they put up two square towers, demolished in 1809, that they used to call "Visigoths' towers". Charles Martel besieged the fortress in 737 and expelled the Saracens, who took the place of the Barbarians.
Time flew... in 1100, the amphitheatre was put in some knights' care, called milites castri arenarum, who kept it until the 14th century: king Charles VI the Mad raised a new fortress a few kilometres away. The knights left, the inhabitants get back their amphitheatre, founded a small city, called "Amphitheatre district", which was still upright in the 19th century.
By the way, did you know king of France François I, who ordered the demolition of all the houses inside the amphitheatre in 1533 when he was in Nîmes city, received from the consuls a small silver arena? Nowadays the amphitheatre houses cultural events.