It was part of Numidia and later became an important Roman colony[2] and an early medieval bishopric (now a Latin titular see) in the casbah area of modern Algiers.

[4]The history of Icosium goes back to around 400 BC when a small Berber village was created by some local fishermen.

They called it y-ksm or ʾy-ksm,[1] which is believed to have meant "seagull's island", and which was eventually transcribed as Icosium in Latin.

[10] Tacfarinas's revolt damaged the city, but Icosium was revived by the introduction of a colony of veteran Roman soldiers during the reign of Juba II.

[14] The Rue de la Marine followed the lines of what used to be a Roman street,[15] and a ruined aqueduct was visible by Algiers's "Gate of Victory" as late as 1845.

The city -of nearly 15000 inhabitants, according to historian Theodore Mommsen- was given full Latin rights by Roman emperor Vespasian.

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. 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.

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.

Inside the lower town, which was densely populated, a network of streets at right angles to each other formed insulae.

The decumanus maximus followed the modern Bab-Azoun street...Of the monuments discovered or noted inside the town, the public baths are of particular importance.