Porta Nigra in Trier, Germany
Porta Nigra (Black Gate) is a Roman city gate built in the year 170 AD.

Trier, which stands on the Moselle river, was a Roman colony from the 1st century A.D. and then a great trading centre beginning in the next century. It became one of the capitals of the Tetrarchy at the end of the 3rd century, when it was known as the 'second Rome'. The number and quality of the surviving monuments are an outstanding testimony to Roman civilization.
(https://web.archive.org/web/20051027113944/http://whc.unesco.org/en/list/367)