The Middle Ages was a crucial period for the city and left traces of great beauty and of great interest. Entire historical districts were built during that period, such as Monti and Trastevere.
In the Constantinian era the Basilicas of the Holy Cross in Jerusalem and St. John Lateran were built.
Other churches were created on the tombs of the martyrs, like the one of San Sebastian on Via Appia, San Lorenzo on Via Tiburtina, Santi Marcellino e Pietro on Via Labicana, Sant’Agnese Fuori Le Mura on Via Nomentana and St. Peter’s Basilica itself in the Vatican.
Later arose the Basilica of St. Paul outside the walls and that of Santa Maria Maggiore; then the Churches of Santi Giovanni and Paolo, San Vitale, San Marco, San Lorenzo in Damaso, Sant’Anastasia, Santa Sabina, San Pietro in Vincoli, San Clemente and Santo Stefano Rotondo.
The lack of a real army to defend the city made it very vulnerable to invasions and raids. For a long time many of these buildings suffered attacks and looting, including the most unfortunately devastating ones in the 5th century by the Visigoths and Vandals. Alongside these disasters were also famines, pestilences and floods of the Tiber.
In Carolingian times, Rome had a revival under Pope Adrian I. An embankment was built along the Tiber and Palazzo di San Giovanni in Laterano was restored and enlarged. The churches were decorated with mosaics of great value, for example those of Santa Cecilia in Trastevere and Santi Quattro Coronati.
Around the year 1000, Rome was devastated by the Saracen raids. St. Peter’s Basilica was sacked in the 9th century and Pope Leo IV decided to build defensive walls. These Leonine Walls were constructed next to Castel Sant’Angelo.
Later, with the emergence of the great papal families, who often fought each other, Rome was divided into zones of influence and new districts were born, each under the control of a local ruler. Towers were erected and new palaces were built on the area of the seven hills. The city was divided and organized into Rioni (districts) with their own banners and, often, their own armies.
In 1330 Pope Boniface VIII instituted the Jubilee that attracted crowds of pilgrims from all over the known world. After him the papacy moved to Avignon and the decadence of Rome was relentless.
In 1377, with the return of Pope Gregory XI, the city, then plagued by conflicts and turmoil, began a new slow journey of rebirth and, in the 15th century, it began to flourish once again under Pope Martin V of the Colonna family.