Icosium's Greek name Ikósion was later explained as deriving from the Greek word for "twenty", supposedly because it had been founded by twenty companions of Heracles when he visited the Atlas Mountains during his labors.
Punic colony
In fact, the settlement and its name seem to have come from Punic colonists at some time before the 3rd century. Its Punic name was then transcribed into Greek and Latin. Punic Icosium was only a small trading post.
Roman town
In 146, Icosium became part of the Roman Empire. Tacfarinas's revolt damaged the city, but Icosium was revived by the introduction of a colony of veteran Roman soldiers during the reign of. The city was given Latin rights by the emperor Vespasian. Roman Icosium existed on what was the "marine quarter" of the city of Algiers until 1940. The Rue de la Marine followed the lines of what used to be a Roman street, and a ruined aqueduct was visible by Algiers's "Gate of Victory" as late as 1845. Roman cemeteries existed near Bab-el-Oued and Bab Azoun. Under the Romans, there were also other settlements nearby on the banks of the Haratob. By the 2nd century, an influx of Berbers from the countryside changed the settlement's demographics, so that Latin-speakers became a minority elite. Christianity started to be practiced in the late 2nd century, and in the early 4th century was the main religion of the local Romanised Berbers in the city. The bishops of Icosium are mentioned as late as the 5th century. At the Christian council of Carthage in 419 went the bishop Laurentius "Icositanus", as representative of Mauretania Caesariensis: Saint Augustine wrote about him in a letter to Pope CelestineI.
Later history
Icosium remained part of the Roman Empire until it was conquered by Vandals in 430. In 442, an agreement between the Roman Empire and the Vandals allowed Icosium to be occupied by the Romans during the Vandal control of northern Mauretania Caesariensis. Some berber tribes took control of the city at the beginning of the 6th century, but the town was later reconquered by the Byzantine Empire. This happened just before the Arab conquest in the late 7th century. Icosium was then destroyed by the Arabs and reduced to a very small village in the 8th century. Most of the romanized inhabitants were killed or sent as slaves to Damascus. Until 950, only ruins remained of the Roman Icosium. Only in the 10th century started to be again developed by Buluggin ibn Ziri to what is now the capital of modern Algeria. Indeed the Casbah of Algiers is founded mainly on the ruins of old Icosium. It is a mid-sized city which, built on a hill, goes down towards the sea and is divided in two: the High city and the Low city, that now are dangerously crumbling
Religion
Around 400, a Diocese of Icosium was established under Roman rule, which was suppressed around 500, presumably by the Arian Vandals. In 1700, the diocese was nominally restored as titular bishopric of Icosium. On 10 August 1838 the titular see ceased to exist as the residential diocese was restored under the city's modern name as Roman Catholic Diocese of Algiers, which was promoted on 25 July 1866 as Metropolitan Archdiocese of Algiers.
