This is one of several turrets remaining from the time when Luxembourg was a fortified city.
They are usually referred to as "Spanish turrets" because they were built in the late 17th century during the time when Luxembourg was under Spanish rule.
It is thought that there were once 32 of these turrets, although this is only an informed guess.
Quite a few can still be found around the fortification areas, most of which were put in place by French military architect Vauban.
Their purpose is not sufficiently understood, but it seems they were used as outlooks for the garrison.
The French under Louis XIV were so impressed by the site that Luxembourg gained the designation of "Gibraltar of the North" as it was perceived to be practically impenetrable.