Built in the third century as part of Rome's defense system, this massive fortification wall is a must see. The wall was commissioned by the Emperor Aurelian after the city's previous structure was in ruins. The wall has undergone significant restoration, and the best-preserved gate is that to the Via Appia, or Appian Way, known as Porta San Sebastiano. If you visit the Museum of the Roman Walls (at 18 Via Porta San Sebastiano, Tue-Sun 9am-2pm) located in the gate you can usually gain access to the wall and get a matchless view of the surrounding area, little of which has changed from when the Apostle Peter traveled through Rome on foot. The Appian Way is the Roman Republic's oldest highway, with sections still in use today. BUS: 118 from Circo Massimo metro station or the hop-on hop-off Archeobus from in front of Stazione Termini (see www.trambus.com)