Porta Nigra

The best-preserved Roman city gate north of the Alps, located in Trier, Germany. Built around 170 AD from approximately 7,200 sandstone blocks weighing up to 6 tonnes each, held together by iron clamps. The name 'Porta Nigra' (Black Gate) originated in the Middle Ages as the stone darkened; its Roman name is lost. In the 11th century, the hermit monk Simeon inhabited the east tower, leading to its conversion into a church -- which ultimately saved it from destruction. Napoleon ordered its restoration to Roman form in 1804. Part of the UNESCO World Heritage Site since 1986.