The Porta Nigra is a large Roman city gate in Trier, Germany. It is today the largest Roman city gate north of the Alps.
The Porta Nigra was built in grey sandstone after 170 AD. The original gate consisted of two four-storeyed towers, projecting as near semicircles 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. For example, the stones at the northern (outer) side of the gate were never abraded, and the protruding stones would have made it impossible to install movable gates. Nonetheless, the gate was used as a town entrance for centuries until the end of the Roman era in Trier.
