Ever heard of Trajan’s bridge over the Danube?

In The Foreign King, my upcoming sequel to The Exiled Queen, the governor of Moesia Superior pitches the idea of a permanent bridge over the Danube to Emperor Trajan at the beginning of the second century CE. Rome has been locked in a decades-long war with Dhawosia, a fictional kingdom inspired by ancient Dacia (the precursor to modern-day Romania). Trajan must succeed where his predecessor, Domitian, failed, but the Dhawosians, who live north of the Danube, fight hard to keep their homeland free of the Roman yoke. And not just metaphorically: Dhawosian gods explicitly forbid enslavement, while the Romans enslave every nation they conquer.

The most amazing ancient bridge

The plan to build this permanent bridge feels like sacrilege from the Dhawosian north shore, faith in divine favor and ethnic superiority from the Roman one, and good hydrodynamics to our modern eyes. A few facts that hooked me when I studied this bridge (I first learned about it in elementary school):

• Location and scale: Downstream from Viminacium at Drobeta, the bridge spanned more than 1,100 meters. That’s about five times the length of the Tower Bridge in London.
• The Iron Gates: Two thousand years ago, the Danube ran a slightly different course through a sometimes-different geography. At the Iron Gates, it raged through rock-cut gorges (called “boiling cauldrons” by locals) so fiercely that navigation was impossible. You might remember that in The Exiled Queen, Prince Dapyx crosses the Iron Gates in his quest to save Queen Andrada. I loved writing that sequence, though it took months of research and drafting. And I knew I wasn’t done telling the story of the river back then.
• The sandbar: The Roman architect Apollodorus of Damascus (fictionalized in my novels as Apollonius of Damascus) discovers a submerged sandbar. It’s just enough to split the river’s flow in half, which gives him options beyond cofferdams and fighting the strong current. This discovery drives my bridge-building plotline and sets the novel’s ticking clock.

In my novel-in-progress, the Dhawosians try to stop the Romans at every turn. Their king hits supply lines and launches night strikes on the scaffolding. He loses some of these skirmishes because holding terrain is easier than advancing. The Romans don’t need to conquer the country yet. They just need to keep the job site alive.

Wait, they did that in 103 CE?

In my historical fantasy, I let the alternate history go wherever the story takes it. But here’s what I learned from studying the real bridge:

• Longest of its age, built in just two seasons: Contemporary writers and archaeologists agree it was completed super fast: about two years from first pile to first march. For a structure that dominated river engineering for a millennium, that speed was extraordinary.
• Twenty colossal stone piers, a wooden superstructure: Ancient accounts (Cassius Dio) describe around 20 masonry piers (broad enough for ice and flood, tall enough to keep the roadway high and dry), carrying a timber-arch deck. Stone for permanence, wood for replaceable strength: a war bridge the Romans could repair between campaigns.
• Roman mortar: Far from the volcanic sands of Campania, they still used hydraulic mortars—lime mixed with crushed brick and additives—to set underwater or damp foundations. Add iron clamps poured with lead, and you get masonry that resists scour, frost, and centuries of floods.
• Forts at both ends: The bridgeheads were part customs house, part consecrated monument to the Roman gods. A Roman fort stood at each end, with access to the bridge only through a military camp.
• Big enough for two wagons to pass: Ancient sources noted the bridge’s impressive width. That meant fast, two-way traffic for troops, supplies, and artillery across a river that had thwarted imperial armies for generations.
• Everyone kept talking about the bridge: Trajan’s Column shows scenes of river works and the spirit of the Danube watching. Later writers (Procopius) still marveled at the remaining piers. Underwater archaeology in the 20th century found pier stumps and even preserved timbers at low water. When modern dams changed the river, traces of the ancient bridge were still there.
• It changed the war: A permanent crossing became a supply artery. Once the bridge opened, Trajan could push columns north, feed them reliably, and end the exhausting chase through gorges that had favored Dacia for decades.
• Propaganda on coins: Rome minted sesterces showing the Danube bridge beneath a triumphal arch. Every hand that touched those coins received a message: The emperor masters rivers, the gods favor Rome, and the old frontier is now a road that expands the empire north.
• Designed to survive winter, then dismantled by policy: The high deck and cutwaters helped the bridge withstand ice runs, but after the conquest of Dacia, Emperor Hadrian removed the wooden superstructure (leaving the massive piers) so enemies couldn’t use the bridge against the empire.

Trajan’s bridge was a turning point in the Roman Empire’s history of conquest and expansion. I hope this gives you a sense of why it works as the centerpiece of my dramatic plot in The Foreign King.