The Bock in Luxembourg City is the former site of the Castle of Lucilinburhu (built in 963), and what eventually became Luxembourg City sprouted up around it. The Bock casemates are composed of a vast latticework of tunnels built into the rock itself, and were built gradually over a century (1644 to 1746). Used as a fortress by the Spanish, the French, and then the Austrians (everyone in Europe it seems at one point had a claim on this bit of land), the caves were also used as a bomb shelter during the second world war.
The casemates surround a bridge we frequently use in Luxembourg City, and we had previously walked around the grounds themselves when my parents visited this past Christmas (more photos here). However, they are only open to the public during the warmer months from March to October. They seemed to open a little later this year for some reason (June), but we finally were able to explore them from the inside. At only 3 Euros per person, it’s definitely a must-see attraction.
My only warning would be to those with heart or musculoskeletal issues that would prevent you from walking or climbing long distances; it’s also probably not a good idea if you are claustrophobic. There are a lot of dark, narrow passageways, and a lot of climbing on steep stairs. But if that doesn’t intimidate you, you can’t miss out. This walk through Luxembourg’s history provides you with some great views of the city from the windows of the fortress, and–if the heat of summer is getting to you–it is much cooler inside.
Even those with issues who can’t climb around the majority of dark passageways can come through the main entrance and take in some of the views from the main hall. Just be careful on those stairs! Some views from in and around the casemates:
For more information, here is a link to the attraction on Trip Advisor.