Icosium was first a Phoenician, then a Roman city whose site is now occupied by the casbah area of the modern city of Algiers. According to Greek legend, Icosium was founded by 20 companions of Hercules, the Greek name, Ικοσιον, being claimed to derive from εικοσι, the Greek word for twenty.
In fact, however, the settlement was occupied by Punic settlers from at least as early as the 3rd century BC. They called it Yksm, which is believed to have meant "owl's island", and which was eventually transcribed as Icosium in Latin. The original Punic name is reflected in the modern Arabic name for Algiers (Arabic: الجزائر, pronounced Al Jaza'ir), which means "the islands".
Icosium remained a small trading post in the Phoenician and Carthaginian periods. In 146 BC, Icosium became part of the Roman Empire. The berber revolt of Tacfarinas damaged the city, but soon Icosium was repopulated with some roman colonists. The city was given Latin rights by emperor Vespasian. Roman Icosium existed on what is now the marine quarter of the city of Algiers. The rue de la Marine follows the lines of what used to be a Roman street. Roman cemeteries existed near Bab-el-Oued and Bab Azoun.
Algiers presents but few Roman remains; and it is still uncertain what name it bore under Latin sway, some thinking it "Icosium", and others Jomnium. Mr. Blofeld says that there are Roman ruins on the banks of the Savus (Haratob), south-east of Algiers; and he thinks this more probably the site of Icosium than Algiers. Mr. Berbrugger mentions the remains of a Roman via, Rue de la Marine, near the port of the capital, which he thinks must have corresponded in most respects with the old Moorish harbour before 1830. Mr. S. Marie informs us that at the quarter of the Gate of Victory, in the old town, there stood on one side of the gate, in 1845, a fountain of white marble, constructed among the ruins of a Roman aqueduct. — John Reynell Morell
Christianity started to be worshipped 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 AD (promoted by Saint Aurelius) went the bishop Laurentius "Icositanus", as representative of Mauretania Caesariensis: Saint Augustine wrote about him in his letter 209, sent to Pope Celestine I.
It remained part of the Roman Empire until the 5th century AD, when it was conquered by Vandals in 430 AD. But in 442 AD, 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 Eastern Roman Empire (Byzantium). It happened just before the Arab conquest in the late 7th century. Icosium was then destroyed and reduced to a very small village in the 8th century. Only in the 10th century started to be developed by Buluggin ibn Ziri to what is now the capital of modern Algeria.
Indeed the Casbah of Algiers (a world heritage site of the UNESCO) 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 
- Casbah of Algiers
- Mauretania Caesariensis
- Roman 'Coloniae' in Berber Africa
- Romano-Berber states
- Christian Berbers
- Detailed map of Mauretania Caesariensis
- Roots of Algiers (in French)
- Edward Lipiński (2004). Itineraria Phoenicia. ISBN 90-429-1344-4.
- Map of ancient remains in the marine quarter of Algiers
- El Djazaı̈r: histoire d'une cité d'Icosium à Alger
- Diocese of Icosium
- Lettera 209, 8 (in Italian)
- Smithsonian: Save the Casbah
- Devoulx Albert, Bedredine Belkadi, Mustapha Benhamouche. El Djazaı̈r: histoire d'une cité d'Icosium à Alger. Publisher ENAG. Algiers, 2003 ISBN 9961623193
- Laffi, Umberto. Colonie e municipi nello Stato romano Ed. di Storia e Letteratura. Roma, 2007 ISBN 8884983509
- Mommsen, Theodore. The Provinces of the Roman Empire Section: Roman Africa. (Leipzig 1865; London 1866; London: Macmillan 1909; reprint New York 1996) Barnes & Noble. New York, 1996
- Reynell Morell, John. Algeria: The Topography and History, Political, Social, and Natural, of French Africa. Publisher N. Cooke. London, 1854 ( )
- Smyth Vereker, Charles. Scenes in the Sunny South: Including the Atlas Mountains and the Oases of the Sahara in Algeria. Volume 2. Publisher Longmans, Green, and Company. University of Wisconsin. Madison,1871 ( Algiers, Roman Icosium )