[1] Frederick intervened when the Guelph party in the city expelled his garrison from the city, forcing the men, led by the imperial vicar Simeon, count of Chieti, to withdraw into the rocca of San Lorenzo.

Frederick was reached by the news at Melfi, and immediately raised an army to set the matter, while the rebel citizens of Viterbo requested help from Frederick's traditional enemy, the pope.

[2] The attack of the emperor had been concentrated on the valley near where the later Porta Faul was erected, and where the then Castle of San Lorenzo stood with less protective walls.

[3] Pope Innocent IV, fearing that the event could start a war with the emperor, intervened; his legate, Cardinal Otto of San Nicola in Carcere, convinced the rebels to sign a treaty of peace.

However, after the signature of the treaty, the troops in Viterbo treacherously attacked and massacred the imperial garrison and Ghibelline followers.