5,400–7,200+ 300+ The siege of Tortona in 1155 was the first major military engagement resulting from Holy Roman Emperor Frederick Barbarossa's ambition to enforce Imperial hegemony in Italy.

Frederick began his first Italian campaign in October 1154, allegedly to attack the Norman Kingdom of Sicily in accordance with an agreement with the Pope.

Upon encountering resistance and insubordination from the Northern Italian city states, his ostensible subjects, Frederick declared a preliminary Imperial ban on Milan in December 1154.

[2] The citadel of Tortona, situated on a mountain, held a commanding position over the Po valley and was protected by massive walls and fortified towers.

[3] The besiegers used an engine to bore a tunnel to undermine one of the towers, but the defenders dug a counter-tunnel that collapsed the effort and killed some of the miners.