Well, the original theatre that stood on this spot was built during the Hellenistic era. Later, it was rebuilt by the Romans, sometime between AD 41 and AD 117. It was capable of seating 25,000 people and was built with each level pitched higher then the previous one, improving the view and the acoustics. Considering that the population of the city was around 25,000 at its height, that means about 10% of the population could sit in the theatre at one time. It looks like an amazing place to watch a performance.