The first underground tunnels were built in 1644, in the era of the Spanish domination. The 23-kilometer long galleries were enlarged only 40 years later by Vauban, the French military engineer and fortress builder, and in the eighteenth century by the Austrians. The subterranean defensive passages were placed on different levels and reached down as far as 40 meters. It is these impressive defense works that conferred Luxembourg the name of "Gibraltar of the North". After the dismantling of the fortress in 1867, 17 kilometers of the casemates were spared, left in good condition. Since 1933 the Bock and Pétrusse casemates have been open to the public.
Depicted in this photo is the main gallery of the Bock casemates, opening up to cannon chambers with loopholes on the right. These underground passages, dug under the Bock promontory, had space for 50 cannons and a garrison of a few hundred men.
- Source: www.lcto.lu/pdf/casemates_en.pdf
Best viewed Large, On Black (although the photo is not really sharp).