Connected to the western part of Castel Grande, the bulwark completely blocked off the Ticino valley, guaranteeing the Duchy of Milan control of numerous alpine passes.
Although the origins of this mighty wall are not precisely documented, it is likely that an early barrier wall was erected in the aftermath of the conquest of Bellinzona Castle by the Visconti family in 1340. But it was only during the 1480s, that is, in the era of the Sforza family, that the bulwark was significantly reinforced and expanded in light of the growing expansionist aims of the Confederate. The complex was doubled through the construction of a second wall, thus creating the two passageways that can still be seen: one covered, about 4 meters high, and the other, in the form of an upper external walkway, equipped with battlements and embrasures.
In addition to forming a structure that served to defend and keep watch over the territory, the bulwark – along with its gate – was also an important point for controlling goods in transit to Bellinzona and other regional fairs. In the fifteenth century, the bulwark had its own castellan, who – not always successfully – watched out to ensure that the walls were not crossed for military or smuggling purposes.
In 1489, by order of Duke Ludovico Sforza, a bridge was also built – also called the Ponte della Torretta – with the intention of further closing off the valley from Castel Grande to the right bank of the Ticino river. This imposing fortified system – about 800 meters long – was severely damaged by the flood of the Ticino river in May 1515 – an event also known as the “Buzza di Biasca” – which destroyed the bridge and a stretch of the bulwark itself about 150 meters long.