Inhabited since very ancient times (between 5000-3000 BC), l'Île-Rousse was in 1000 BC a small, prosperous town called Agilla; it was dependent on Tyre in Phoenicia. Destroyed by the Phoenician fleet of Calaris (Galeria), Agilla came under Roman rule as Rubico Rocega (red rock) until the 4th century AD. Being so close to the sea, it was threatened by pirates and other potential enemies, and was not inhabited for several centuries except for fishermen and peasants who lived on the products of the sea and earth around the villages of Santa Reparata and Monticello.
In the 17th century, merchants from Santa-Reparata-di-Balagna established shops to trade by sea with the coastal villages of the Balagne, Nebbio, and the west of the Cap Corse. About 1759, Pasquale Paoli, who often came to Balagne, decided to equip Corsica with a port in the northwest of the island to try to cut the sea traffic between Genoa and Calvi. His plans prepared, he persuaded the Balagne government, sitting in Algajola, to authorize him to create a fort protecting the port (the Scalu) on 10 December 1765. L’Île-Rousse was born from this decision