The Coliseum
The Romans were famous for their grand pieces of architecture and spectacle and nothing was more spectacular than The Coliseum of Rome.
Used to house Gladiator fights, Plays, Public Addresses; the Coliseum was at the very epicenter of Roman culture. A replica of it was built at Camp Jupiter and another replica is still somewhat being built in Tertio Rome. The Tertio Rome version of the Coliseum is pretty much identical and is just having a lot of the finer details finished on it but with the Greeks over the River it is presumed that the stadium will be used a lot more for rival sporting occasions and other important business.
The Coliseum houses 50,000 people it is enchanted to have perfect acoustics for concerts and plays, and it can even change the turfing of the arena floor to match whatever it is being used for thanks to a magical dial in the Emperor's (Praetors') Box.
There is also a bast network of underground tunnels and chambers underneath the Coliseum to house weapons, props and animals.