The valley park follows the Pétrusse stream beneath the great viaducts of the old town. Below: gentle paths, lawns, footbridges, and the small Saint-Quirin Chapel cut directly into the cliff (the oldest place of worship in Luxembourg). Above: the cliffs and ramparts you have been walking on at street level.
A complete loop from Place de la Constitution back to the city centre takes under an hour and is the best way to grasp how the topography shaped the old fortress. Lovely in autumn.