When people mention a city, we immediately think and connect that with an image (either building or structure). Like for example - New York City: Empire State Building, San Francisco: Golden Gate Bridge, Paris: Eiffel Tower. You get my point. And when you hear Rome, you think of THE COLOSSEUM.  

You cannot pass up the chance to see the Colosseum when you are in Rome. Gladiators + Arena = Are. You. Not. Entertained?