Modern Description:
Trajan's Bridge or Bridge of Apollodorus over the Danube was a Roman segmental arch bridge, the first bridge to be built over the lower Danube. Though it was only functional for a few decades, for more than 1,000 years it was the longest arch bridge in both total and span length. The bridge was constructed in 105 AD by instruction of Emperor Trajan by Greek architect Apollodorus of Damascus for the deployment of Roman troops during the conquest of Dacia. The bridge was situated East of the Iron Gates, near the present-day cities of Drobeta-Turnu Severin in Romania and Kladovo in Serbia. The structure was 1,135 m (3,724 ft) long (the Danube is now 800 m (2,600 ft) wide in that area), 15 m (49 ft) wide, and 19 m (62 ft) high, measured from the surface of the river. At each end was a Roman castrum, each built around an entrance, so that crossing the bridge was possible only by walking through the camps. The bridge's engineer, Apollodorus of Damascus, used wooden arches, each spanning 38 m (125 ft), set on twenty masonry pillars made of bricks, mortar, and pozzolana cement.[2][3] It was built unusually quickly (between 103 and 105), employing the construction of a wooden caisson for each pier. Engineers waited for a low water level to dig a canal, west of the modern downtown of Kladovo. The water was redirected 2 km (1.2 mi) downstream from the construction site, through the lowland of Ključ region (sr), to the location of the modern village of Mala Vrbica. Wooden pillars were founded into the river bed in a rectangular layout, which served as the base for the supporting piers, which were coated with clay. The hollow piers were filled with stones held together by mortar, while from the outside they were built around with Roman bricks. The bricks can still be found around the village of Kostol. The piers were 44.46 m (145.9 ft) tall, 17.78 m (58.3 ft) wide and 50.38 m (165.3 ft) apart. Members of the Roman legions and cohorts which participated in the construction of the bridge carved the names of their units into the bricks. Thus, it is known that work was done by the legions of IV Flavia Felix, VII Claudia, V Macedonica and XIII Gemina and the cohorts of I Cretum, II Hispanorum, III Brittonum and I Antiochensium. (https://en.wikipedia.org/wiki/Trajan%27s_Bridge)
Wikipedia: https://en.wikipedia.org/wiki/Trajan%27s_BridgeWikidata ID: Q849786Trismegistos Geo: 29680
Info: Wikipedia.org 
(Wikipedia)
