Trier: The Porta Nigra, viewed from outside
The Porta Nigra (black gate) in Trier, Germany, is a large Roman gatehouse with two four-storied towers, projecting as near semi-circles on the outer side. The structure is largely of grey sandstone, and was built between 180 and 200. In the Middle Ages, the building was used as a church, one of the towers being adapted to contain an apse.
The name Porta Nigra or black gate comes from the layer of black soot that covered the gate during the industrial revolution. This is best seen when the structure is wet from fresh rain. Although the top of one tower is missing, the gate is still remarkable. The inside is open allowing tourists to enter and climb the floors.