The Porta Nigra, also called Porta by the locals, is a sizable Roman city gate located in Trier, Germany. A UNESCO World Heritage Site, that is. Because of the darker color of the stones, Porta Nigra got its name in the Middle Ages; the Roman term has not survived. Grey sandstone was used to build the Porta Nigra after 170 AD. Two four-story towers projecting as near semicircles on the exterior made up the original gate. On either side of the two gate openings was a small courtyard. Unknown fact ... (Read More)