So you are walking on a road in Rome and you suddenly come across this building on your left. What do you think of immediately. "Oh! It's the Colosseum!" you remark, only slowly realising that the building while looking a lot like the Colosseum from some points is also quite different from it. Congratulations! You have found your way to the Theatre of Marcellus- in its heyday – one of the three main theatres of Rome.
The theatre of Marcellus does not have a beautiful story. It was originally started by Julius Caesar who wanted to build a theatre that would rival/ outshine another theatre in existence in Rome- the theatre of Pompey (Caesar's rival- not to be confused with the town Pompeii). But Caesar was assassinated and the theatre stood incomplete. Augustus started the project once again- for Marcellus- his nephew, son-in-law and one-time heir apparent. Except Marcellus died (as did a lot of heirs of Augustus- conspiracy theories abound that Augustus' wife Livia poisoned them all in favour of her own son but I have not read enough to make up my mind one way or the other). But the theatre was completed anyway and inaugurated in around 13-11 BC. It was capable of seating 20,500 people (ref: Amanda Claridge, Rome: An Oxford Archaeological Guide).Via http://optimusmaximusadventurous.wordpress.com/2012/08/21/a-theatre-a-temple-part-iii-of-ancient-roman-sights-in-rome/