Ancient Roman amphitheaters
Although the names "Circus", "Theatre" and "Amphitheatre" are often confused it is worth noting that they were quite distinct buildings used for different types of public event. In the earliest days these buildings were constructed of wood.
The Amphitheatre was specially developed to hold events such as shows of Gladiators and wild beasts and was therefore quite distinct from the Circus, where chariot races were held. The structure of the Amphitheatre was close to that of two common theatres joined back to back, with a central stage called "arena" and elevated rows of seats around it. It looked very similar to a modern football stadium or bullfighting ring.
The name Amphitheatre in fact means "double theatre", which is exactly what it was. So much so that there is an account by Casalius of a man called Curio who on occasion of his father's death built a double theatre out of wood. The two theatres were hinged so that they could be swung round and be joined into a single amphitheatre when necessary.
The Colosseum will now be used to provide a more detailed description of what the amphitheatre could be like although clearly it was the greatest and most advanced and other Amphitheatres wouldn't quite have the same level of infrastructure.