Cencius built a high tower on the bridge opposite St. Angelo and made all who would pass it pay toll to him; but the pope, the famous Hildebrand who was Gregory VII, excommunicated him. He was forcibly seized at the altar and imprisoned until the people came to his rescue. Gregory next fell foul of the emperor and there was a fresh struggle in which the Countess Matilda of Tuscany, a powerful princess, sided with and supported the pope. In the end Henry triumphed, Gregory withdrew into St. Angelo, and from its battlements saw the city sacked and set on fire. Several popes and antipopes then held the castle in turn; it was a constant bone of contention between the powerful factions, and its fortifications were continually strengthened so that whoever was in possession generally dominated the city. Toward the end of the twelfth century a pure spirit, that of Arnold of Brescia, arose to denounce the outrageous misconduct of many of the clerical hierarchy, from the supreme head down to the lesser members of the priesthood; and he exposed in vigorous language their profligacy, reckless ambition and tyranny. The high character and unimpeachable virtue of Arnold gave him much influence and the Church rallied all its weight to crush him, but for some time in vain. He fought strenuously for the revival of the old liberties and to exclude the popes from civil government, but the Emperor Frederick with his army made Arnold prisoner, and he was cast into the dungeon of St. Angelo to leave it only to be hanged in the square before the castle. A fierce struggle now ensued between the emperor and the praetors of the Roman senate recently reconstituted. The battle ground was the Leonine city, in front of and around St. Angelo. Victory inclined in turn to each side. At this epoch Rome was terrorised by the nobles. Issuing from their strongholds--the palaces which they had converted into fortresses--they robbed and pillaged on all sides and forcibly seized citizens whom they held for ransom. The city was depopulated; whole districts lay in ruins, vineyards and vegetable gardens were planted round the Pantheon and the Porta del Popolo. Every one fought for his own. The senate barricaded itself in the Capitol; the pope was not safe outside his castle of St. Angelo; the great nobles, representing powerful families, claimed their independence and relied upon their strength. The Frangipani were established upon the island in the Tiber and held the Colosseum, the arches of Titus, Constantine and Janus, and the Circus Maximus; the Orsini were masters of the quarter surrounding the Vatican; the Savelli held the district where the Cancelleria now stands; the Pierleoni occupied the theatre of Marcellus and the quarter of the Gheto; the Colonna were supreme in the district between the Piazza del Popolo and the Quirinal and were also fortified in the mausoleum of Augustus; on the slopes of the Quirinal were the Pandolfi, the Capocci and the Conti.