Aqueduct of Segovia, Spain
Built in the second half of 1 A.D. during the reign of Roman emperor Trajan, the Aqueduct of Segovia is one of the best-preserved ancient aqueducts. It was built of about 24,000 granite blocks and, surprisingly, no mortar. The two-tier structure has 165 arches, some of them more than 30 feet high. Fani Kurti/Getty Images Arch of Constantine, Italy
Erected in honor of Constantine the Great’s victory over Maxentius in 312 A.D., this magnificent monument is the largest Roman triumphal arch. It is 66 feet high and about 82 feet wide, featuring architectural moldings and relief sculptures of goddesses and emperors.