Pons Alcantarae
About
Gaius Julius Lacer built this granite bridge across the Tagus between 104 and 106 CE, financed by eleven nearby Lusitanian towns and dedicated to Trajan. Its six arches carry the roadway 194 meters across the gorge and rise 45 meters above the water, fitted from massive blocks without mortar. An inscription boasts its builder raised a bridge that would last forever, and in 117 CE it was barely a decade old, already carrying imperial traffic. It still carries traffic nineteen centuries later.
What happened here
Piers are set on cofferdams — a ring of driven piles, pumped dry, filled with concrete that sets under water. They are built with a cutwater pointing upstream to split the current and the ice. The span above may be stone arches or timber on stone, and timber is not a poor relation: it is cheaper, faster, and replaceable. Roads approach on an embankment so the crossing stays level. Tolls are common, shrines at either end are usual, and the inscription names whoever paid. Floods are the enemy; a bridge is inspected after every big one, and the scour around the piers is packed with rubble. A bridge fixes a town's position for a thousand years, which is why so many are still standing.
Sources
Roman Maps shows the empire frozen at 11 August 117 CE, the day Trajan died — its moment of greatest territorial extent. Explore the full map →