Build a Wall or a Bridge

Sometimes you can do both.

Hadrian's Wall "was a defensive fortification in the Roman province of Britannia, begun in 122 AD during the reign of the emperor Hadrian. It ran from the banks of the River Tyne near the North Sea to the Solway Firth on the Irish Sea. It had a stone base and a stone wall. There were milecastles with two turrets in between. There was a fort about every five Roman miles. From north to south, the wall comprised a ditch, wall, military way and vallum (another ditch with adjoining mounds). It is thought that the milecastles were staffed with static garrisons, whereas the forts had fighting garrisons of infantry and cavalry. In addition to the wall's defensive military role, its gates may have been used as customs posts."

The Bridge of Angels "is a Roman bridge in Rome, Italy, completed in 134 AD by Roman Emperor Hadrian, to span the Tiber, from the city center to his newly constructed mausoleum, now the towering Castel Sant'Angelo. The bridge is faced with travertine marble and spans the Tiber with five arches, three of which are Roman; it was approached by means of ramp from the river."