The Porta Nigra is the largest Roman city gate north of the Alps, and it has been standing, in one form or another, for roughly eighteen centuries. Built in the second century from huge blocks of sandstone cut so precisely that they were held together with iron clamps instead of mortar, it once formed the northern entrance to Augusta Treverorum, the Roman city on the Moselle. The stone has weathered to a dark grey, almost black, which gave rise to the medieval name, the black gate, though the Romans themselves would have seen a pale, honey-coloured building.
It survived because of a hermit. In the eleventh century a Greek monk, Simeon, walled himself into the east tower and lived there until his death, and after he was made a saint, the gate was turned into a church, which saved it from being dismantled for building stone like so many other Roman buildings. Napoleon had the church fittings removed in the early nineteenth century, restoring something closer to the original look. You can climb the towers and walk through the upper galleries, which give views across the old town, and a small admission fee is charged.
The gate sits in a pedestrian zone, and Trier's centre is best seen on foot. Leave the bike at one of the parking areas near the edge of the old town and walk along the Simeonstrasse, where cafés and bakeries line the way to the main market. With over a hundred thousand inhabitants, Trier has no shortage of fuel, shops and places to eat, and a university keeps the evenings lively.
The surroundings are a draw for riders. Vineyards cover the steep hillsides of the Moselle, and roads wind along the river toward Bernkastel-Kues and Cochem, with views at every bend. In autumn the vines turn gold and red, and the hillsides glow. Spring brings blossom, and summer is warm, though weekends see heavy traffic.
Karl Marx was born in Trier in 1818, and a house on the Brückenstrasse, now a museum, is a short walk from the gate. The city makes a good base for a few days, especially if you also want to cross into Luxembourg, which is only a short ride away.