Luxembourg has always been a natural fortress, due to the cliffs and river that surround the city. But over the centuries many man-made fortifications have been added, the remains of which are now UNESCO listed.
To explore the fortifications, head to the location of the original fortress in Luxembourg, near Casemates du Bock. Then take either of the two circular walks that start here: Vauban Luxembourg or Wenzel Luxembourg. The latter is my favourite as it passes by this beautiful stretch of the Alzette River. A brochure for either walk can be picked up at the City Tourist Office or your hotel.