The area between the Janiculum Hill and the bank of the Tiber, though a business center and vitally important to Rome's defenses, had always been considered to be outside the city until the era of the Emperor Augustus. The inhabitants of this quarter had always been very cosmopolitan and a number of religions were practiced

the strong Jewish presence here consequently led to the early appearance of Christianity. There was a lot of building activity in this area in the Middle Ages but Trastevere went through a quiet period while the Popes were living in exile in Avignon. The upturn arrived with Sixtus IV and Julius II who created road links by building Via della Lungara, Via Giulia, the Ponte Sisto and the planned but never constructed Ponte Giulio -this project tended to direct the flow of pilgrims along the right bank of the Tiber.
Another significant work of infrastructure was undertaken by Pope Urban VIII. After the 1527 sacking of Rome, he realized how vulnerable this whole area was, so he enclosed it with walls that extended to the already strongly fortified Vatican.
Originally Piazza San Cosimato was the Emperor Augustus's Naumachia, the place where mock naval battles took place. Today's piazza is named after what was once a ninth century Benedictine monastery, today integrated into the Regina Margherita Hospital and one of the most vivacious parts of Trastevere.