The Roman Empire expanded up through all of Germany and by the 1st century BCE had conquered the Celtic tribe of the Treveri. Augusta Treverorum (now Trier) was founded around 16 BCE and was distinguished by its name from other Roman cities in honor of Emperor Augustus. The city became the capital of Roman Belgic Gaul, and later the capital of the Gauls, controlling much of the western Roman Empire.

The first Roman wooden Moselle bridge existed from 18-17 BCE. It was a pile bridge. As dendrochronological studies have shown, the first stone bridge was built in 45 CE, not far downstream from the current location of the Roman bridge.

The Roman (in German Römerbrücke) or Old Moselle Bridge standing today is the third bridge over the Moselle River at this site since the founding of the city and the oldest bridge in Germany at the ancient ford across the Moselle. The piers were laid in the riverbed using watertight sheet pile foundations made of basalt and blue stone blocks. The building material came, among other sources, from the mining area around the former volcano Hohe Buche (in the eastern part of the Eifel). The bridge has pointed upstream supports to better withstand floods and ice. In Roman times, a wooden bridge structure, called a truss, was built on the massive piers. It was able to withstand traffic well thanks to a roadway ten meters wide. Since at normal water levels the roadway was about 14 meters above the Moselle, ship masts no longer had to be lowered when sailing downstream. Due to the strong current, ships had to be towed upstream.

The bridge was built between 144 and 152 CE; much of the original Romerbrucke structure has survived to this day, although some parts — in particular, the road and its arches — date from the 18th century. Built from black basalt of the Eifel mountains, the 9-pier Romerbrucke was erected at the intersection of roads and waterways in the Roman main road system. From this bridge, residents made offerings to river gods, making the Moselle riverbed at this location an important historical and archaeological site. The nine bridge piers were built in the 2nd century CE during the period when Trier was part of the Roman Empire and are still considered a "technical marvel" as they have supported the bridge for two thousand years. The location of the bridge gates Porta Inclyta (Famous Gates), demolished in the Middle Ages, is unclear.

The stone arch was created only in the Middle Ages between 1190 and 1490, possibly under Elector Baldwin. All nine bridge piers have survived to this day; the old assumption that two of these nine were extended in 1717/1718 is incorrect. The first two piers on the city side were hidden on the embankment after reconstruction during Roman times.

In 1689, the bridge was blown up by French troops, and between 1716 and 1718, Trier’s electoral court carpenter Johann Georg Juda renovated the arch. On this occasion, a crucifix and a statue of Saint Nicholas of Myra were installed on the fifth pier from the west.

In 1806, the western bridge gates were demolished, and in 1869, the eastern ones.

In 1931, the bridge was widened and received the current cantilevered pedestrian walkways.

On the morning of March 2, 1945, the Americans were able to cross the intact Roman Bridge heading toward Trier-West. For unknown reasons, the bridge was not blown up by German troops. According to an eyewitness, there was already so much debris on the bridge that the cables used to detonate the explosives were probably damaged.

From the riverbank, the layers of the bridge’s history are easy to identify — the reconstructed 18th-century arches made of red brick stand out against the ancient basalt ones.

Sources:

https://en.wikipedia.org/wiki/Roman_Bridge_(Trier)

https://commons.wikimedia.org/wiki/File:Aerial_view_of_the_Roman_Bridge_at_Trier,_Germany,_circa_in_1956.jpg