Fort de Tournoux, Military fortress in Ubaye Valley, France.

Fort de Tournoux is a defensive complex built in tiers up a steep mountainside in the Ubaye valley. It comprises two long, rectangular, pale-beige stone buildings with dark grey pitched roofs: a larger lower building on the left, below a terraced stone retaining wall, and a slightly smaller upper building to its right, set higher. Both facades have rows of small, evenly spaced rectangular windows and small arched openings. Curved stone vaulted structures and arched passageways are embedded in the rocky slope below and between the buildings.

Construction of this mountain fortress began in 1843 and continued until 1865, built in response to French concerns about potential Italian attacks. In the 1880s, the installation was equipped with modern cannons to strengthen its defensive capabilities.

The fort has been likened to a "military Versailles of the 19th century", the "Eiffel Tower of the Ubaye" and a miniature Great Wall, and to a Tibetan monastery on the mountainside. Its name comes from the hamlet of Tournoux, on a plateau above the Ubaye about two kilometres north of the works. Part of the complex is open to visitors.

Climbing requires good physical fitness because of the many stairs and the mountain setting in the Ubaye valley. After safety improvements in 2016, a section of the fortress is now accessible to visitors, particularly the lower battery section.

The fort's construction was budgeted at 1.5 million francs, but rising costs, extensions and building obstacles pushed the total above 3.5 million francs, straining the defence budget. The unfinished programme was reinforced in 1880 by the battery of Caures and in 1890 by the redoubt of Serre-de-l'Aut, which increased its firepower.