While stationed in Germany with the USAF, I was able to visit some amazing historic landmarks. The Porta Nigra had the most impact on me by far. To stand there and realize how many important people had stood in that very spot was awesome.

"The oldest defensive structure in Germany, the Porta Nigra was erected in about 180 AD when the Roman city was surrounded by walls. Trier was a Roman colony from the 1st century AD and then a great trading centre beginning in the second century. It became one of the imperial capitals under the Tetrarchy at the end of the 3rd century, and became known as the "second Rome."

The Porta Nigra is the only one of four Roman gates that still stands in Trier; the others were gradually pillaged for their stone and iron. The Porta Nigra survived because it was used as the humble residence of a hermit monk named Simeon for seven years (1028-35). After his death he was buried in the gate and the structure was transformed into the two-story Church of St. Simeon (lay church on the bottom, monastery church on top).

Napoleon destroyed the church in 1803, but the 12th-century Romanesque apse survived and the entire structure has been restored to its medieval appearance."

Taken from http://www.sacred-destinations.com/germany/trier-porta-nigra.htm