Forts de l'Esseillon, Military fortification complex in Aussois, France.

Forts de l'Esseillon is a fortification system of five separate forts positioned along a rocky ridge at about 1500 m (4,900 ft) elevation in Savoie. They are spread across the natural rock formation and work together as an integrated defensive network.

Built between 1819 and 1834, the forts were financed by French war indemnities after the 1815 Congress of Vienna and erected under Austrian pressure to shield the Kingdom of Sardinia from a French attack through the Mont Cenis pass. They replaced fortifications destroyed in the 1796 Italian campaign. The Franco-Sardinian alliance of 1857 made them obsolete; after the 1860 annexation of Savoy, France kept all but Fort Charles-Félix, which was partly demolished, and reversed their guns toward Italy.

The five works carry the names of House of Savoy members: Victor-Emmanuel, Charles-Albert, Charles-Félix, Marie-Christine and the Marie-Thérèse redoubt. Marie-Christine, the highest, now holds a cultural centre, a mountain hostel and one of the Vanoise national park entrances; Marie-Thérèse houses a fortification interpretation centre and artist exhibitions. Since 1972 volunteers of the Association des Forts de l'Esseillon have restored the site.

The forts are accessible on foot year-round, though the walking routes vary in difficulty and can take several hours depending on which ones you visit. Winter snow and ice can make conditions challenging, so sturdy footwear and good preparation are needed.

The Marie-Thérèse Redoubt now operates as an interpretation center displaying exhibitions about fortification design and military strategy. Visitors can learn how these mountain defenses were engineered and what made them effective against potential threats.