Remains of a former lock/dam of the Fortress Luxembourg, in Grund/Gronn, over the Alzette river. Its aim was to set the valley upstream under water, so to prevent an attack from there.
<a href="//commons.wikimedia.org/wiki/User:Zinneke" title="User:Zinneke">Zinneke</a>
CC BY-SA 3.0 lu
<span class="int-own-work" lang="en">Own work</span>