The longest ancient Roman bridge was built between 103 and 105 AD across the Ister (Danube) by order of Emperor Trajan. The design of this grand structure was entrusted to Apollodorus of Damascus, one of the most famous architects of that time. The bridge was constructed to provide logistical support for the second campaign against the Dacians during the military campaign of 105-106 AD. The length of the bridge was 1,135 meters, and it was fortified with fortress structures on both sides. Judging by the relief depiction of the bridge on Trajan's Column in Rome, the crossing had a wooden roadway structure mounted on stone piers. The width of the bridge was 20 meters, with a height of 45 meters.

Evidence of the bridge's existence includes a medal struck by Trajan in honor of its completion,

and sesterce coins issued during the emperor's lifetime,

the bas-relief on Trajan's Column in Rome, as well as brief notes from eyewitnesses. From all these sources, it can be concluded that the bridge was arched. Procopius and Cassius Dio provide minor information about the materials used in its construction. According to Cassius Dio's description, Trajan's bridge over the Ister, as the Danube was then called, consisted of 20 piers made of hewn stones, each 45 meters high excluding the foundations, and 18 meters thick. These were spaced 51 meters apart and connected by arches. Theophilus noted that large cofferdams were used for the foundations and piers of the bridge, which were filled with a mixture of large stones and cement.

The bridge was destroyed just half a century after its construction by order of Emperor Hadrian, to prevent hostile tribes from penetrating the right-bank Roman territory. According to Cassius Dio: "Hadrian, fearing that the barbarians, after killing the garrison, would gain access to Moesia, destroyed the bridge's roadway." As for the fate of the architect himself, Hadrian destroyed Apollodorus just as he did his bridge. After a quarrel with the emperor, the architect was executed, possibly due to disagreements over the wisdom and premature destruction of his greatest construction.

The absence of the bridge significantly hindered Trajan from completing the conquest of Dacia during the first Dacian war. The lack of a permanent crossing prevented the Romans from quickly transferring siege equipment, supplies, and troops to the left bank of the Danube. However, in 101 AD, the Roman army crossed the Danube without difficulty in two columns via pontoon bridges near Drobeta. There was no need to transport siege weapons to the walls of the Dacian fortresses, as the forest-rich Dacia allowed legionaries to manufacture them directly at the battlefield. Thus, the construction of such a magnificent structure lacked any practical purpose, including military and logistical. The sole motivation for building the bridge over the Danube by Emperor Trajan was to demonstrate military power and technical superiority to the enemy, as well as to make an indelible impression on neighboring peoples and tribal unions.

In the summer of 105 AD, Trajan inspected Apollodorus's bridge, built by the labor of soldiers from the VII Claudian Legion during 103-105 AD. Shortly before this, the Romans repelled a Dacian attack near the crossing. When Trajan illuminated the bridge, six foreign delegations awaited the emperor's arrival. Dacian envoys, on behalf of King Decebalus, promised peace on the condition that the Romans would not start a war. Also present were envoys of the Germanic Suebi, representatives of Greek states including the Bosporan Kingdom, and a delegation from distant India. Historian Cassius Dio saw the bridge about 70 years after its construction. By that time, the upper elements had already been removed by order of Emperor Hadrian. Cassius Dio describes Trajan's Danube bridge as follows: "Trajan built a stone bridge over the Ister, about which I do not know how to adequately express admiration for this emperor. There are other remarkable structures of his, but this one surpasses them... How can one not marvel at the expenses spent on this construction? How can one not marvel at the method by which each pier was built in the middle of a multi-water river, abundant with whirlpools, on muddy ground? After all, it was impossible to change the river's flow."

The bridge's piers are located on the Danube near the cities of Drobeta-Turnu Severin (Romania) and Kladovo (Serbia). In 1856, a record low water level revealed all 20 piers.

They obstructed river navigation, and after lengthy debates, two piers were demolished in 1906.

By 1932, 16 piers remained,

and in 1982 archaeologists recorded only 12.

Sources:

https://dzen.ru/a/ZG8SYL-Pv3ZcoM5e

https://dic.academic.ru/dic.nsf/ruwiki/661408

https://dic.academic.ru/dic.nsf/ruwiki/661408

http://history.niv.ru/doc/kovalev-rome-lectures/rome-lectures-35.htm