Porta Nigra

The best-preserved Roman city gate north of the Alps, and the anchor of the best Roman city in Germany. Trier's Porta Nigra was built from huge gray sandstone blocks after 170 AD, and it survived because a Greek monk, Simeon, lived in it as a hermit from 1028, and after his death the gate was turned into churches, with a monastery beside it in his honor. Climb inside for the views, then keep going: Trier also has Constantine's basilica hall, the Imperial Baths and an amphitheater. It is the place to understand the late empire, when this city was one of the residences of the Western Roman emperors.