In today's terms, it's hard to fathom how much of the world the Romans once controlled. At its peak, the Roman Empire spanned from Hadrian's Wall in Scotland to Morocco, from the banks of the Rhine River bordering Germania, to Egypt, and east to the Euphrates River in Mesopotamia—a huge swath of conquered peoples and occupied lands, with barbarians on the borders.
Thousands of years later, monuments of Roman glory and imperialism still stand throughout Europe. Huge concrete buildings like the Colosseum remain, as do stone plinths and obelisks.
But the Romans' most impressive achievement, and most important contribution to modern Europe, lies underfoot.
Roads, built to allow the empire to flow outward, and for the rewards of empire to come flooding back to the capital, were the key to the Romans' governance of Europe. Along these roads ran messengers, as a type of precursor to the American Pony Express—a relay of horsemen could carry a message 50 miles a day. Governors, emperors, legions, and, most importantly, trade flowed along these ribbons of stone that cut through hills and across gorges. In 9 BCE, the Emperor Tiberius rode almost 215 miles along the roads in 24 hours to get to his dying brother's side. For a small toll, other, non-official travelers could travel the roads as well.
Roman roads are still visible across Europe. Some are built over by national highway systems, while others still have their original cobbles—including some of the roads considered by the Romans themselves to be the most important of their system.
One major road you can still visit is via Appia, or Appian Way, the most strategically important of the Roman roads. Begun in 312 BCE, the road runs from Rome southeast to the coastal city of Brindisi, a distance of 350 miles. It was via Appia that allowed for the Roman conquest of southern Italy, and the defeat of the Greek city-states and colonies embedded there.
The Appian Way was host to battles between the Romans and the Greek general Pyrrhus from 280 to 275 BCE, which is where we get the term "Pyrrhic victory:" a victory that comes at too high a cost. Via Appia was also the site where, in 71 BCE, around 6000 members of Spartacus's slave army were crucified on the hillsides.
In France (Gaul) and Spain (Hispania), the road systems were no less impressive than in Italy—the Alcantara Bridge, over the Tagus River, was built in 106 AD, and carries the inscription "Pontem perpetui mansurum in saecula, " or "I have built a bridge which will last forever." This is not strictly true. While the bridge still stands, it was damaged, first by Moors in 1214, then by the Spanish in 1762, and again in 1809. There are, in fact, two bridges in Spain named Alcantara, built by the Romans, and a third, similarly named bridge, the Alconetar Bridge—all of which span the Tagus, in Western Spain. The word "Alcantara" comes from the Arabic word "al-qantarah", which means "bridge".
The Romans also acquired roadways and Latinized them—the via Domitia, which ran between Italy and Hispania through southern France, was an ancient path that the Romans paved in 118 BCE. Prior to the Roman upgrade, the path featured in Greek mythology as the legendary route of Heracles when he drove the cattle he stole from Geryon along it. This road was also the one travelled by the Carthaginian general Hannibal when he invaded Italy in 218 BCE. Along the via Domitia lie the remains of two bridges, one of which, the Pont Julien, is passable to foot traffic. The other, the Pont Ambroix, is ruined but beautiful.
Along the Roman road via Julia Augusta, which also runs through the south of France, there are the remains of a number of bridges. The Pont Flavien is in the best condition, and is perhaps the most archaeologically important: it is the only surviving example of its kind of bridge, and was constructed as a funerary monument. It was heavily used until the end of the 20th century, and has been extensively repaired. Pont Flavien has sustained a lot of damage in the last few hundred years—part of it collapsed in the 18th century, and, during the Second World War, both a German tank and an Allied truck crashed into one of the archways. The bridge is now reserved for foot traffic.
Then there are the world famous archways of the aqueduct that crosses the Gardon River, the Pont du Gard. Built in the first century, the aqueduct and bridge stand in such good condition due to their being used as toll roads during the Middle Ages. Originally the bridge was used for vehicular and pedestrian traffic, but access is now restricted to pedestrians.