[1] Pope Gregory VII was besieged in the Castel Sant'Angelo by the Holy Roman Emperor Henry IV in June 1083.
He held out and called for aid from Guiscard, who was then fighting the Byzantine Emperor Alexios I Komnenos in the Balkans.
He entered Rome and forced Henry to retreat, but a riot of the citizens led to a three days sack, after which Guiscard escorted the pope to the Lateran.
After days of unending violence, the Romans rose up and caused the Normans to set fire to the city.
Many of the buildings of Rome were gutted on the Capitoline and Palatine hills along with the area between the Colosseum and the Lateran.