The Ossola territories, in the area to the northwest of Milan, have constituted the western border most in contact with the nordic and tens-alpine world, ever since the first establishment of the Duchy of Milan. It is already known from G. Parker’s monography on the camino español that one of the common routes, which allowed overland redeployment of Spanish troops headed towards Flanders, from the Liguria region across central Europe, would go through Ossola and cross the Simplon Pass or the Gries Pass. During the turbulent historical period of the Thirty Years’ War and the following one, the changing fortunes of the Duchy of Milan in Spanish hands led to the fast and strategic conquest of Piedmontese cities (1639) and their equally rapid loss on the western border. Especially in the second half of the seventeenth century, the Franco-Savoy advance threw the Piedmontese borders into a severe crisis and the Spanish governors of Milan accordingly adopted all the military measures needed to address the issue. Fearing incursions from the north, through Romandie, Valais and Ossola, in the late seventeenth century, many field engineers among whom Beretta and Formenti, arranged the transformation of Domodossola, the outermost military stronghold only equipped with obsolete medieval walls at the time, into a “modern” rampart city (1687-1690). The engineers produced an accurate study of the territory, preserved today in the Historical Civic Archive and at the Trivulziana library in Milan in a cartographic manuscript series of all the Ossola valleys and the Swiss territory from Brig to Lake Leman.