I have tried to make a few corrections --
The Fenestrelle Fort is the biggest fort in Europe, having a surface area of 1.300.000m2. Built from 1720 to 1850, this fort is located between altitudes 1.800 and 2.435 m. The Fenestrelle Fort guards the access to the Fenestrelle valley, which was acquired in 1713 by the Kingdom of Sardinia.
History
France ceded Fenestrelle to Sardinia, as required by the Treaty of Utrecht in 1713. To defend the newly acquired territories, a complex structure of forts was built. -- <by whom?>
These forts were connected by a 3-km long wall and a long indoor stair of 4,000 steps.
The fort was never under siege; instead, it was used as a prison by the French Consulate and the French Empire. Notable prisoners held at this fort include Joseph de Maistre, Bartolomeo Pacca, several political prisoners, Mazzini's supporters, and the Archbishop Luigi Fransoni.
In 1861, after the unification of Italy, around 24,000 supporters of the Kingdom of the Two Sicilies (mainly soldiers) were imprisoned in the fort. The fort, therefore, became a concentration camp. Several Garibaldi and Papal States supporters were also locked up in this fort. Most of the prisoners died of hunger and cold.
In 1882, the fort was upgraded. After 1887, it became the headquarters of the Fenestrelle bataillon of the Third Alpini Regiment. During Fascism, this fort was again used as a prison.
After World War II, the fort was abandoned and left to decay. Everything that could be carried away was plundered.
In 1990, a redevelopment action guided by a group of volunteers was started to restore this fort.
