The reaction of the region's Gauls was swift; they drove the colonists off the lands. Taking refuge in Mutina, the latter sent for military assistance. A small force under Lucius Manlius was prevented from reaching the area. The Senate then sent two legions under Gaius Atelius. Collecting Manlius and the colonists, they descended on Piacenza and Cremona and successfully placed castra there of to support the building of the city. Piacenza must have been walled immediately, as the walls were in place when the Battle of the Trebia was fought around the city in December. There is no evidence either textual or archaeological of a prior settlement at that exact location; however, the site would have been obliterated by construction. Piacenza was the 53rd colony to be placed by Rome since its foundation. It was the first among the Gauls of the Po valley.