The Fenestrelle Fortress, better known as the Fenestrelle Fort is a fortress overlooking Fenestrelle (Piedmont, northern Italy). It is the biggest alpine fortification in Europe, having a surface area of 1,300,000 m². Originally built by the French to a design by Vauban beginning in 1694 and enlarged by Savoy between 1728 and 1850, it guards the access to Turin via the Chisone valley, and stands at altitudes between 1,100 and 1,800 m. The territory was acquired in 1709 by the Duchy of Savoy (later known as the Kingdom of Sardinia) after the defeat of the French at fort Mutin (Fenestrelle).