AW San Carlo, Fortress in Airolo, Switzerland

AW San Carlo is a Swiss artillery fortress on the Gotthard Pass above Airolo, at 2060 m below Lago di San Carlo and the Lucendro dam. Built into rock, it has 690 m of tunnels and 4,000 m² of underground floor space, with five caverns holding freestanding buildings on a pedestal grid. Two 10.5 cm armoured turrets, model 39 L52, were mounted by K+W Thun.

Construction began in 1938 as a prototype for a new generation of fortresses with two 10.5 cm armoured turrets, after Italy built a road to the San Giacomo Pass in 1929 just 14 km from the Gotthard tunnel portal. Both turrets were operational by November 1939, the quarters were finished in 1942, and the site remained in service until 1999.

The site's name comes from the San Carlo curve of the old Gotthard road beside it. From 1948 to 1987 a semi-mobile battery of four heavy 12 cm mortars stood here, and five infantry bunkers with machine guns guarded the approaches. The fortress company trained its soldiers in mountaineering, with mountain guides and ski instructors.

The fortress stands at 2060 m above sea level on the Gotthard Pass, below Lago di San Carlo and the Lucendro dam, at the lower end of the Valletta di San Gotthardo by the San Carlo curve of the old Gotthard road. The converted site reopened as the seminar and wellness hotel La Claustra in 2004; after a 2010 closure, Rainer Geissmann of Airolo bought it at auction for 1000 francs and reopened it in September 2015.

The fortress's two 10.5 cm armoured turrets, model 39 L52, were mounted by K+W Thun and connected in November 1939, before the barracks were finished in 1942. The guns could fire in all directions up to 22 km, reaching 24 km with pointed shells.