[2] The site of the battle was the field between a small hill and the walls of the city of Tusculum, at a place called "Prataporci", about 25 km southeast of Rome.

In his universal chronicle, the Chronica Universalis, the contemporary writer Sicard of Cremona describes the site of battle as "near Monte Porzio" (apud Montem Portium).

It did not work: when the consul (leader) of the Roman commune learned of the arrival of Rainald at Tusculum, he sent an army to besiege the archbishop in the city.

The communal Roman army refused Christian's diplomatic overtures and instead attacked with their whole force, numbering 10,000 poorly armed men, on Whitsunday.

In the year 1166 since the birth of Christ, Emperor Frederick, after settling the conflict between the princes, as we have mentioned, and restoring good order to the situation in Germany, assembled an army from all parts of the empire and led it into Italy, crossing the Alps for the fourth time.

When this was reported in Rome by messengers, the Romans, whose strength was estimated as 30,000 armed men, moved out from the entire city and suddenly besieged the archbishop in the castle, to the dishonor of the emperor.

But the Romans themselves, completely unlike the ancients in this and all other respects, answered that they would not grant his request but arrogantly threatened that on this day they would give him and his entire army to the birds of heaven and the wild animals of the earth to eat.

But the archbishop, completely unshaken by the answer he had received from them – for he was not inexperienced in the troubles of war – with great energy encouraged his men for battle by promises and threats.

He warned them in noble words that they could not place their hope in flight, since they were too distant from their fatherland and the emperor's army to be able to flee, but, mindful of their inherent courage and of the cowardice that was natural to their enemies, they should fight for their lives with all their strength.

After the battle had begun and the lances were broken at the first clash of the armies, the fight was carried on with swords, while the archers on both sides obscured the light of day with their arrows as if they were snow flakes.

While the Romans therefore were fighting only with the weight of their mass, Bishop Christian with his men penetrated their battle line from the flank, tore the middle of their formation apart, and covered with blows the enemy that was thus skillfully separated into three groups.