As you are walking through the valley and look up toward the city center, you see the Bock - what's left of the fortified promontory that surrounded the old city.
Part of this promontory is called the Bock Casemates. The casemates are the remains of underground passages that were used for military defense purposes in previous centuries.
|the holes in the rock above are lookout/canon spots in the casemates|
When we wound our way back to the top of the city, we found ourselves at the entrance to the Casemates, so we decided to go in and look around (about 3 Euros each). Apparently it is the most popular tourist attraction in Luxembourg, so why not?
|tunnels. lots of tunnels.|
|a bridge between sections|
*Not pictured: dark, narrow spiral staircases, several dead ends, dead pigeon.
It was pretty cool and definitely worth doing just for the unique experience (as long as you're not claustrophobic). I'd say it's somewhat kid-friendly if you avoid the staircases. You can read more about the casemates here.