ancient Roman city south of Hadrumetum (modern Sousse) in what is now Tunisia. Although it was originally a native community influenced by Carthaginian civilization, Thysdrus probably received Julius Caesar's veterans as settlers in 45 BC. Thysdrus did not become a municipium (settlement with partial rights of citizenship) until the reign of Septimius Severus (AD 193–211). In 244 it finally became a Roman colonia (settlement with full rights of citizenship). The wealth of its territory is shown by a vast amphitheatre built there, the largest Roman monument in Africa and second in impressiveness only to the Colosseum at Rome. Because the site lies under the modern town of El Jem, excavation is difficult.