Sperrstelle Chlus, Military fortress in Graubünden, Switzerland.

Sperrstelle Chlus is a military fortress in the mountainous region of Graubünden built into rock with multiple bunkers and tunnel systems. The installation was designed as a defensive position with several levels containing artillery positions and ammunition storage areas.

The fortress was built between 1937 and 1942 as part of Swiss defense strategy against possible invasions during World War II. It served as a blocking position to protect the strategically important mountain valley and was one of many similar works along the country's borders.

The site's name refers to the Chlus gorge, the narrow access to the Prättigau. The infantry works Felsenbach Nord A 6308 and Süd A 6309 guarded the gorge, while the artillery work Nussloch A 6330 in the Mastrils rock face provided fire support. The Felsenbach Süd work was cleared and liquidated in 2016, so its interior is no longer intact.

The fortress is accessible only through guided visits since many areas have narrow and low passages. Those with mobility restrictions should check beforehand which sections of the installation are accessible.

Inside the fortress is an approximately 160-meter-long infantry barracks carved entirely from rock. This hollow space provided room for soldiers, ammunition, and equipment all within one protected location.