Trier, Germany

Founded by Romans in 16 BC as Augusta Treverorum, Trier stands as the oldest city in Germany, home to roughly 112,000 residents along the Moselle River in Rhineland-Palatinate. The 180 AD Porta Nigra city gate, a 7,200-ton structure measuring 36 meters by 21.5 meters, ranks as the largest Roman building north of the Alps. UNESCO-listed sites include the 320 Imperial Roman Baths and the 360 AD Basilica of Constantine, the largest surviving Roman hall at 67 meters. The 1270 Cathedral of Saint Peter holds the Holy Tunic relic of Jesus, and Karl Marx, born here in 1818, is now commemorated at a museum.