The Porta Nigra (Black Gate) was built in Trier, Germany in the 2nd century and is the largest city gate known to us from the Roman period.  The massive, castle-like structure was originally made of light sandstones, that got their present dark color as a result of aging and pollution.  It was this darkening that led to the name "Porta Nigra". 

The gate dates back to a time when the Romans often erected public buildings with huge stone blocks.  The stones were cut by bronze saws powered by mill wheels 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.  In 1016 the gate was converted into a twin church, dedicated to St. Simeon.  Emperor Napoleon I had these additions demolished in 1804-19, and the gateway was restored to its original form - leaving only the apse.  Inside, traces of the double church are still visible.

During this restoration the upper story of the eastern tower was destroyed.  This is the only real damage to the stone gate.  There are still remnants around Trier of what was once a 4 miles long city wall connecting to the gate..

Trier, on the Mosel River and located on the border with France and Luxembourg, is Germany's oldest city.  It was a settlement in 2000 BC and was conquered by the Romans.  The Romans left a lot of buildings in Trier, including Porta Nigra and 3 bath houses.