At the entrance to the old town and now next to the famous shopping street "Simeonstraße"", stands the Porta Nigra, a fortified Roman gateway whose truly colossal dimensions make it a unique monument of its kind and time. It was built of the distinctive light sandstone, by placing one row of huge blocks on top of another, and then joining them with iron clamps without using any mortar. Its construction dates from the 2nd century A.D., when Trier, until then an open town, was surrounded by walls with their towers and gates. The Porta Nigra actually served a dual purpose: as a strong and effective defence and as an equally effective and impressive symbol of might and power. Its height is 30 meters and its depth 22 meters, while its front measures 36 meters in length. It has 2 carriage ways, each 7 meters high, leading through a windowless ground floor. But there are in all 144 windows with Roman arches on the first and second floors at the front, and looking across the inner court, provided with portcullis and secured by heavily strengthened gates. The two towers flanking the Porta Nigra, while built flush with the rest of the walls inside the courtyard, jutted outwards in massive curves at the front, where they faced the open fields, and were linked with the high walls (7 meters) surrounding the town, covering an area of 28500 acres. With its roughly hewn blocks of sandstone, now weatherbeaten, darkened by the smoke of centuries, and shapeless through wilful destruction, it still retains its former impressive effect of massive strength. The fact that this gate to the north had its equal counterpart to the town's southern entrance, and a similar gate to the east, and another, where the Old Bridge crosses the Moselle, strengthening this ring of walls, give a fairly accurate idea of how the power of Rome and its planning on a gigantic scale, gave the town its very shape. Thanks to the conversion of the upper storeys of the gate and the Collegiate Curch of St. Simeon into a twin church (11th cent.), in his memory, this Roman monument was preserved to prosterity.