In the paper are discussed the likely causes of the collapse and the temporary safety features adopted to secure a collapsed portion of the Historic Walls of the city of Pistoia (Italy). The walls of medieval origin, affected by various subsequent modifications and alterations, have been subjected to a sudden collapse, after a period of heavy rain, for a length of about 25 meters. Hereby are described the probable causes of the collapse, due to a poor quality of the walls (external façades with external layer made up of river pebbles and inner filling material in-coherent), height of building is not modest (approximately eleven meters), a significant differential in height between the two faces (about four meters) and a not unlikely rotation of the base. It is proposed a technique of temporary reinforcement with a system of steel tie-beams and braces with a view to the work of reconstruction of the partially collapsed and consolidation of the lateral parts of wall.