Trier, which stands on the Moselle River, was a Roman colony from the 1st century AD and then a great trading centre beginning in the next century. It became one of the capitals of the Tetrarchy at the end of the 3rd century, when it was known as the ‘second Rome’. The number and quality of the surviving monuments are an outstanding testimony to Roman civilization.
Trier is an example of a large Roman capital after the division of the empire. The remains of the Imperial Palace, in addition to the Aula Palatina and the Imperial Thermae, are impressive in their dimensions. The city bears exceptional testimony to Roman civilization owing to the density and the quality of the monuments preserved: the bridge, the remains of the fortified wall, thermae, amphitheatre, storehouses, etc. In particular, funerary art and the craftsmanship of potters, glassworkers, and moneyers flourished in the city.
Sometimes referred to as the 'second Rome', Trier had no claim to this title until the division of the empire by Diocletian in 286 and the institution of the Tetrarchy seven years later. However, even before this era, the Roman city was flourishing. The original centre of the colonial town, the regular insulae, for the most part built during the reign of Claudius (41-54), had extended so much by the mid-2nd century that a wall was built, enclosing the industrial quarters and the nearest thermae (baths) to the south, the amphitheatre, which extended beyond the decumanus maximus to the east, and, most likely, a hippodrome. At the same time, a sandstone and basalt bridge was built over the Moselle, extending westward from the decumanus, which replaced an earlier construction, the foundations of which have been found.
It was between 258 and 268, when Postumus took up residence there in order to foil the threats of the Franks and the Alemans on the limes (frontier) that Trier became a capital for the first time. When Constantius Chlorus, the ruler of Brittany and Gaul since the division of 293, moved there, it gave more permanence to this choice.
The reconstruction of the city, the name of which was changed to Treveris, was then undertaken on a large scale by Constantine the Great after 306. The restored amphitheatre and thermae, the Circus maximus, and what remains of an immense imperial palace, construction of which required the destruction of several insulae, reflect a deliberate political choice that grew out of the new balance established by the Tetrarchy. Trier is directly and tangibly associated with one of the major events of human history, Constantine's march against Maxentius in 312, which was a prelude to the Edict of Milan (313) and which signalled the recognition of Christianity as the official religion of the Roman Empire.
It was in Trier that in 326 Constantine founded the twin basilicas to commemorate his twenty years of power; they live on in the form of the Cathedral and the Church of Our Lady. After the death of the great emperor in 337, Trier was the place of residence of his son, Constantine II, and afterward of Valentinian and Gratian. As well as being the capital of the Empire, Trier was additionally the location of the Prefecture of Gaul, an immense administrative district which stretched from the limes germanicus to the Atlantic and from Hadrian's Wall to Tingitana in Mauritania.
During the reign of Constantine the Great, the primordial role of Trier in the spread of Christianity became manifest. The invasions of the Goths ushered in the decline of Trier. The imperial capital was then moved to Milan, which was the capital of the Gallic Prefecture of Arles. However, the evolution of Trier has been marked by historical continuity. The layout of the city still corresponds to its 2nd-century configuration, with the major thoroughfares of the cardo (Simeonstrasse) and the decumanus (bridge). For a long time, the major monuments were used in their original capacity: for example, the Aula Palatina ,where Constantine gave audiences, became the palatium of the Frankish counts before falling around 1200 into the hands of the Bishops of Trier who, also Prince-Electors, made this great hall a part of their palace between 1615 and 1647. The surviving Roman gate, known as the Porta Nigra, has undergone several changes of use