ancient Roman city, the site of which, at present-day Timgad, on the high plateau north of the Aurès Mountains in northeastern Algeria, offers the most thoroughly excavated and best-preserved Roman remains in North Africa. Thamugadi, founded by the emperor Trajan in AD 100, proved to be of strategic importance in the defense of Numidia. Its long prosperity was derived from the fertility of the surrounding territory. In the late 4th century it became the seat of the bishop Optatus, one of the most ardent supporters of Donatism, a heretical Christian movement, and the stronghold was sacked by Berbers in the early 6th century, toward the end of the Vandal supremacy in Africa.