You can do an enormous amount of your sightseeing on foot. This area was also the heart of the city in the time of imperial Rome, and the most impressive remains are here. There is the Forum - the heart of ancient Rome, the home to the political and religious institutions of the greatest empire the world has seen. There is the Colosseum, awe-inspiring even two millennia after it was first built, and monumentally impressive. And there is the Pantheon, originally a temple built before the birth of Christ, and still remarkably intact.
Cross the Tiber and you are at St Peter's, the world's largest church. And next door lies the Vatican and its incredible complex of museums.
And when you're done with sightseeing, the area is packed with great bars, restaurants, coffee shops ... this is a perfect quarter to sit out at a café, and enjoy the street life that makes Rome so exciting.
