THE ALCANTARA BRIDGE OVER THE TAGUS
Trajan also built a public work of enormous importance, a true investment. The spectacular Alcantara Bridge spanning the Tagus River served to exploit the region's rich mines: thanks to it, gold and silver were transported more quickly to the Mediterranean coast and then to Rome by ship, and were essential to finance the Empire's ever-increasing costs.

Trajan entrusted the bridge's design to the architect Caius Julius Lacer, who completed it in just a few years. At one end, he built a small temple with an inscription proclaiming, "As long as my bridge lasts, the world will last."

So far, he has been right, because both the temple and the bridge (which has a 70-meter-high central arch) still exist and even modern trucks pass over them, proving that Roman public works were designed to last (almost) forever. Only with stone, and without reinforced concrete.

THE FORTIFIED BRIDGE ON THE DANUBE
When Trajan attacked Dacia, he crossed the Danube on a temporary pontoon bridge, which is depicted on Trajan's Column.
But between 103 and 105 AD, the emperor decided to build a more solid masonry bridge, undoubtedly to ensure the army's regular supplies without fearing that river floods would destroy the pontoon bridge or prevent its use.
At the chosen location To build it, the Danube was a full 800 meters wide, so Trajan entrusted this demanding project to the only architect capable of executing it, Apollodorus of Damascus, the aforementioned star architect of the time, who had already built other works for him.

The bridge over the Danube connected the castrum of Drobeta (now in Romania) with the castrum of Pontes (now Kladovo in Serbia), located on the opposite bank. It was over a kilometer long and fifteen meters wide, allowing two carts to pass in each direction and providing wide sidewalks for pedestrians. For centuries, it was the longest bridge in the Roman Empire and probably in the ancient world.

Cassius Dio (155-235 AD) described it thus: "The bridge rests on twenty square stone piers, 150 feet high excluding the foundations, and 60 feet wide. These [pylons] are 170 feet apart and are connected by arches."

Therefore, there were twenty large pylons, shaped to cut through the river's waters, 44 meters high, 50 meters apart, and almost 18 meters wide. Above them rested a wooden structure that, once again, we can reconstruct based on the reliefs on Trajan's Column.

A grandiose work, but already in the time of Hadrian, there were plans to dismantle it because it had become a veritable highway that favored barbarian incursions. The final destruction seems to have occurred later, during the reign of Aurelian (241-275 AD).

In 1856, the level of the Danube dropped significantly, and almost all twenty pylons were exposed, but in the early 1900s, some of them were demolished because they posed a danger to river navigation. Recent archaeological excavations have confirmed the presence of about twenty pylons, one of which has been excavated, isolated from the water, and visible in situ.

The construction system with formwork, concrete, and shaped pylons was certainly imitated by Hadrian when she built the Pons Aelius, which led to her Mausoleum in Rome. We explain this in our book "Castel Sant'Angelo. Mausoleum of Hadrian. Architecture and Light," which reveals its construction secrets, the fruit of centuries-old Etruscan and Roman mastery in hydraulics.