3 Best Sights in Aventino and Testaccio, Rome

A former monastery, founded in the 7th century by monks fleeing Jerusalem following the Arab invasion, this is a major monument of Rome, though it takes on a subdued air thanks to its modern quiet surroundings in the upscale San Saba district. The serene but rustic interior harbors 10th-century frescoes, a famed Cosmatesque mosaic floor, and a hodgepodge of ancient marble pieces.

This Early Christian basilica is stark and tranquil, showing off the lovely simplicity common to churches of its era. Although some of the side chapels were added in the 16th and 17th centuries, the essential form is as Rome's Christians knew it in the 5th century. Most striking are the 24 fluted Corinthian columns that line the classical interior. Once bright with mosaics, today the church has only one above the entrance door (its gold letters announce how the church was founded by Peter of Illyria, "rich for the poor," under Pope Celestine I). The beautifully carved, 5th-century cedar doors to the left of the outside entrance are the oldest of their kind in existence.

One of Rome's most significant churches is a couple of Metro stops farther down Via Ostiense from Testaccio. Built in the 4th century AD by Constantine, over the site where St. Paul had been buried, the church was later enlarged, but in 1823 a fire burned it almost to the ground. Although the location near the river can be dreary and the outside lacks any real charm, the rebuilt St. Paul's is massive, second in size only to St. Peter's Basilica, and has a sort of monumental grandeur that follows the plans of the earlier basilica.

Highlights include the 272 roundels depicting every pope from St. Peter to Pope Francis (found below the ceiling, with spaces left blank for pontiffs to come) and the cloisters (€4, tickets available in the gift shop), where you get a real sense of the magnificence of the original building and a glimpse at artifacts unearthed from early workshops that surrounded the church. In the middle of the nave is the famous baldacchino created by sculptor Arnolfo di Cambio.