The Cremaschi refuse an ultimatum sent to Crema by Frederick on February 2, 1159, asking the destruction of their walls, then settle into their city to hold against a siege.

The siege is marked by several episodes of brutality.

The Germans hang some Cremaschi prisoners to their siege machines, hoping the defenders will not fire against their fellows.

However, this expedient does not work, and turns the battle into a slaughter.

As Barbarossa has killed his prisoners, so the Cremaschi hack their prisoners to pieces in front of their comrades.

The besieging troops are formed mostly by Barbarossa's imperial contingents, part of which are led by his brother, Conrad, and by the latter's son Frederick; by Bavarian troops under duke Henry the Lion; and by communal troops, mostly belonging to the main imperial allies, Cremona (under bishop Oberto of Dovara) and Pavia.