The Romans built arenas,theaters,bridges, aqueducts, and roads more than two thousand years ago.Some of these constructions are still in use to day. Why did they last so long? What wasthe secert of Roman architecture? Here is the answer: One of the Roman's greatest inventions was the rounded arch, called the Roman arch,and another was cement,a mixture of lime,clay, and water. One of the early uses of the Roman arch was for the constrution of aqueducts, which carried water cities. Usually, an aqueducts had several levels of arches. The Romans often placed the larger and wider arches in the bottom row for support, and the smaller and narrower arches were on the top level.Also,on the top, they placed a lead U-shaped pipr along which the water flowed, thanks to the pull of gravity.One of the best-preserved Roman aqueducts is at Pont du Gard in France. It was built in 19 B.C.E.