Fodor's Expert Review Bock
Luxembourg's raison d'être juts dramatically out over the Alzette river valley. This cliff served as the principal approach to the town as far back as Celtic and Roman times, until bridges were constructed. The name comes from the Celtic büück, meaning the promontory supporting a castle. Over its farthest point looms the ruined tower of the castle of Sigfried himself, father of the city. He founded the fortress Lucilinburhuc in 963; it was later expanded from this dominant point by countless invaders until the walls were finally razed in 1875.
The main reason to visit are the labyrinth of underground defensive tunnels, known as the Casemates du Bock. These were first built in 1644 by Spain and were expanded some 40 years later to include 23 km (14 miles) of underground galleries. Tours of the casements are one of the delights of the city.