Timgad (ancient Thamugadi or Thamugadis), ruined city of Algeria, on the slopes of the Aurès Mountains, south of Constantine, and southeast of Batna. It was founded by the Roman emperor Trajan in ad 100 and named, in honor of his sister, Colonia Marciana Trajana Thamugas. The ruins of the city are extensive; it is known as the Numidian Pompeii. The ruins, all of which, except a 7th-century basilica, date from the 2nd century, include the remains of an arch, known as the arch of Trajan; a forum; a library; and a theater having a seating capacity of nearly 4000. Baths with paved and mosaic floors have been found in a perfect state of preservation; numerous large private dwellings have been uncovered, one house sometimes occupying a whole block. The existence of several churches indicates that the city was a center of Christianity in the 3rd century. With the decline of the Roman Empire, Timgad ceased to flourish, and suffered from the invasions of the Vandals, the Byzantines, and the Arabs. The city was forgotten until 1881, when excavations were begun.