It lies in a valley between low vine-covered hills of red sandstone in the west of the state of Rhineland-Palatinate , near the border with Luxembourg and within the important Moselle wine region. Trier was one of the four capitals of the Roman Empire during the Tetrarchy period in the late 3rd and early 4th centuries.
The archbishop-elector of Trier also had great significance as one of the seven electors of the Holy Roman Empire. With an approximate population of ,, Trier is the fourth-largest city in its state, after Mainz , Ludwigshafen , and Koblenz. It is one of the five " central places " of the state of Rhineland-Palatinate. The first traces of human settlement in the area of the city show evidence of linear pottery settlements dating from the early Neolithic period.
Since the last pre-Christian centuries, members of the Celtic tribe of the Treveri settled in the area of today's Trier. The city later became the capital of the province of Belgic Gaul ; after the Diocletian Reforms , it became the capital of the prefecture of the Gauls , overseeing much of the Western Roman Empire. In the 4th century, Trier was one of the largest cities in the Roman Empire with a population around 75, and perhaps as much as , Sometime between and , probably in the Roman administration moved the staff of the Praetorian Prefecture from Trier to Arles.
The city continued to be inhabited but was not as prosperous as before. However, it remained the seat of a governor and had state factories for the production of ballistae and armor and woolen uniforms for the troops , clothing for the civil service, and high-quality garments for the Court.
South of this line, Roman control was firm, as evidenced by the continuing operation of the imperial arms factory at Amiens. The Franks seized Trier from Roman administration in Relics of Saint Matthias brought to the city initiated widespread pilgrimages. The bishops of the city grew increasingly powerful and the Archbishopric of Trier was recognized as an electorate of the empire, one of the most powerful states of Germany.
