The Porta Nigra is the largest Roman city gate north of the Alps built around the 2nd century. It got its name from the darkened color of its stone; the original Roman name has not been preserved. Locals commonly refer to the Porta Nigra simply as Porta. The original gate consisted of two four-storied towers, projecting as near semi-circles on the outer side. A narrow courtyard separated the two gate openings on either side. For unknown reasons, however, the construction of the gate remained unfinished. However, the gate was used for several centuries until the end of the Roman era in Trier.