By the later half of the 2nd century BC, it belonged to the Romans, who included it in their province of Africa, and gave it the name of "Regio Syrtica". Around the beginning of the 3rd century AD, it became known as the Regio Tripolitana, meaning "region of the three cities", namely Oea (i.e., modern Tripoli), Sabratha and Leptis Magna. It was probably raised to the rank of a separate province by Septimius Severus, who was a native of Leptis Magna. In spite of centuries of Roman habitation, the only visible Roman remains, apart from scattered columns and capitals (usually integrated in later buildings), is the Arch of Marcus Aurelius from the 2nd century AD. The fact that Tripoli has been continuously inhabited, unlike e.g., Sabratha and Leptis Magna, has meant that the inhabitants have either quarried material from older buildings (destroying them in the process) or built on top of them, burying them beneath the streets, where they remain largely unexcavated. There is evidence to suggest that the Tripolitania region was in some economic decline during the 5th and 6th centuries, in part due to the political unrest spreading across the Mediterranean world in the wake of the collapse of the Western Roman empire, as well as pressure from the invading Vandals. It is recorded by Ibn Abd al-Hakam that during the siege of Tripoli by a general of the Rashidun Caliphate named Amr ibn al-As, seven of his soldiers from the clan of Madhlij, sub branch of Kinana, unintentionally found a section on the western side of Tripoli beach that was not walled during their hunting routine. Those seven soldiers then managed to infiltrate through this way without being detected by the city guards, then managed to incite a riot within the city while shouting Takbir, causing the confused Byzantine garrison soldiers to think the Muslim forces were already inside in the city and flee towards their ship leaving Tripoli, thus allowing Amr to subdue the city easily. According to al-Baladhuri, Tripoli was, unlike Western North Africa, taken by the Muslims very early after Alexandria, in the 22nd year of the Hijra, that is between 30 November 642 and 18 November 643 AD. Following the conquest, Tripoli was ruled by dynasties based in Cairo, Egypt (first the Fatimids, and later the Mamluks), and Kairouan in Ifriqiya (the Arab Fihrids, Muhallabids and Aghlabid dynasties). For some time it was