Pope Gregory VII was besieged in the Castel Sant'Angelo by the Emperor Henry IV in June 1083. He held out and called for aid from the Guiscard, who was then fighting the Byzantine Emperor Alexius I Comnenus in the Balkans. He returned, however, to Italy and marched north with 36,000 men. He entered Rome and forced Henry to retire, but a riot of the citizens led to a three days sack, after which Guiscard escorted the pope to the Lateran. The Normans had mainly pillaged the old city, which was then one of the richest cities in Italy. After days of unending violence, the Romans rose up causing the Normans to set fire to the city. Many of the buildings of Rome were gutted on the Capitol and Palatine hills along with the area between the Colosseum and the Lateran. In the end the ravaged Roman populace succumbed to the Normans.