The Arçon's lunette and its underground passage

The history of the lunette begins with Vauban’s plans to fortify Mont-Dauphin. During his second visit in 1700, the engineer noted that the site had certain weaknesses, particularly on the Eygliers front. He therefore planned several forward defenses to supplement the main defenses. His plan included, in particular, three redoubts positioned in front of the northern front, intended to monitor the approaches to the stronghold and reduce blind spots.

However, these ambitious plans were not implemented immediately. The kingdom’s financial difficulties and changes in the strategic situation slowed down the work. Following the Treaty of Utrecht in 1713, the border was pushed eastwards and Mont-Dauphin lost its status as a first-line stronghold. It became a second-line stronghold, although its capacity to accommodate a large garrison meant it retained a military and logistical function.

Only one of the three redoubts envisaged by Vauban was eventually built. It was constructed in front of the Eygliers front between 1728 and 1731, under the supervision of the garrison’s engineers. Its purpose was to reduce blind spots along the front and improve surveillance of the surrounding area.

The structure we see today, however, is the result of a later alteration. In 1791, against a backdrop of revolution in which the defense of the borders once again became a major concern, General Jean-Baptiste d’Arçon proposed upgrading this old redoubt. His plan introduced a new type of defensive structure: a lunette with a safety redoubt, designed to continue offering resistance even if the enemy managed to breach the first lines of defense.

The conversion notably included a central tower serving as a security redoubt, as well as casemates for flanking fire. Unlike fire directed outwards, these features made it possible to engage the attacker from the rear and defend the moats once they had already breached some of the obstacles. A cylindrical tower, topped by a conical slate-covered roof, completed the structure.

However, the project encountered numerous difficulties. In 1792, just as work on the security redoubt had begun, a lack of funding forced the contractor to halt the works. The site thus remained unfinished for several years, despite the concerns of the engineers, who warned of the vulnerability created by the interrupted works. The conversion was finally completed in 1803.

Another distinctive feature of the Arçon lunette lies in the way it is accessed. From the stronghold, soldiers can reach the fortification via an underground passage approximately 113 metres long, which emerges into the moat in front of the Bastion du Roi. This passage allowed the defenders to move between Mont-Dauphin and the lunette without having to cross the glacis in the open, where they would be exposed to enemy fire.

Inside, the layout of the structure follows a particularly ingenious defensive design. In particular, galleries facilitate eavesdropping and counter-mining, whilst the casemates with rear-firing embrasures enable defenders to fire on attackers whilst they are in the moats. Storage and rest areas were also provided for the soldiers.

The lunette should therefore not be regarded as a mere small, isolated fortification. It constitutes a genuine forward position, deliberately situated in front of the ramparts. The soldiers occupying it are on the front line, but have a protected route to reach the stronghold should the need arise. This layout perfectly illustrates the principle of Mont-Dauphin’s successive defenses: to slow down the enemy, keep them in the open, maximise firing opportunities, and then allow the defenders to withdraw gradually towards the stronghold.

The Arçon lunette also bears witness to a significant development in military architecture. In Vauban’s time, fortifications no longer sought to rise high like the medieval castles. On the contrary, the development of artillery necessitated lower, thicker structures, protected by substantial earthworks capable of absorbing the impact of cannonballs. Bastions, half-moons, moats and advanced works also made it possible to increase the number of cross-fires and eliminate blind spots.

With the transformation devised by d’Arçon, defense evolved once again. The lunette foreshadowed the advanced fortification systems that would develop in the 19th century: rather than concentrating the entire defense behind a single wall, the aim was gradually to drive the enemy away from the stronghold and to organise several structures capable of providing crossfire. Arçon’s lunette and the two bastions flanking it thus contributed to this evolution in fortification.

Today, the Arçon watchtower retains a unique place in the Mont-Dauphin landscape. It stands as a testament to Vauban’s designs, their adaptation by 18th-century engineers, and the alterations made to the fortifications on the eve of the Revolution and during the Empire. Its underground passage, galleries and ingenious defensive system still offer a practical insight into how military strategists conceived and organised the defense of a mountain stronghold.

The Lunette d’Arçon is now open to the public as part of guided tours of Mont-Dauphin, which include a visit to its 113-metre-long underground passage. It forms part of the remarkable fortified complex designed according to Vauban’s principles, which has been listed as a UNESCO World Heritage Site since 2008.