The 1st Rome's Colisseum. It's smaller than the famous one but is awesome like it.
Not to be mistaken for the Colosseum, this is only a very short walk from Piazza Venezia, so it should definitely be something to be seen.
it is less than 10 minutes walking
Similar to Colosseum.
Originally built in the early years of Imperial Rome. As many of the Roman monuments it has had a long and interesting life, with it starting as a theatre and later serving as a fortress.