For the newly arrived guest, the Porta Nigra is the best place to begin a tour of Trier. The gate dates back to a time (about A.D. 180) when the Romans often erected public buildings of huge stone blocks (here, the biggest weigh up to six metric tons).
The slabs were cut by bronze saws powered by mill wheels (some cutting traces are still visible) and put together without mortar. Instead, two stones each were held together horizontally by iron clamps whose bent ends were embedded in corresponding holes by molten lead. One clamp is visible inside the gate near the eastern spiral staircase; rust traces can be seen in many holes on the outside because in the Middle Ages people chiseled these holes to retrieve the metal for recycling.
The stone blocks were spared a recycling because of the Greek monk Simeon, who had himself walled up in the eastern tower as a hermit after 1028. After his death in 1034/5, he was buried inside the gate and made a saint. In his honor, two churches were built into the gate (torn down 1804-1819). The upper story of the eastern tower was razed - the only real damage to the stone gate, whose name, »Black Gate,« is medieval and goes back to the black pollution patina on the gray sandstone.
Inside (fee), traces of the double church, Roman stone masons' marks, and date inscriptions are visible.
Further information: trier-info.de/english/porta-nigra-info