Roman Holiday is one of the most important set in Rome and where the Colosseum is the protagonist, along with many other beauties of this city. This film is a 1953 American romantic comedy film directed and produced by William Wyler.  The stars are Audrey Hepburn, as a princess who wants to visit Rome, and Gregory Peck as a reporter. Hepburn won an Academy Award for Best Actress for her performance; the screenplay and costume design also won.  

The official trailer:

Ann, a crown princess from an unnamed European nation, is on a state visit to Rome, becomes frustrated with her tightly scheduled life, and secretly leaves her country's embassy. The delayed effect of a sedative makes her fall asleep on a bench, where Joe Bradley, an expatriate reporter for the "American News Service", finds her, without recognizing who she is.   He convinces her to spend the day with him. They visit the Mouth of Truth and later tour the Colosseum.  

A clip shot on the famous staircase of Piazza di Spagna:

The clip on the Mouth of Truth