Within the boundaries of present-day Paris, the first inhabitants settled on the Ile de la Cité. After several invasions, Paris’ first Gallo-Roman city wall was built in the 4th century to repel the barbarians. Expansion to the right bank began in the 5th century, near Saint-Gervais. To protect the Ile de la Cité more effectively from invasion, the Châtelet fortress was built by Louis VI in the 12th century, before becoming redundant in the 13th century with the construction of Philippe Auguste’s enceinte.
The Ile Saint-Louis was originally called the Ile Notre-Dame, and was used for grazing and storage. When Philippe Auguste’s enceinte was built, most of the 4th arrondissement became part of Paris. In the 14th century, Charles V’s enceinte completely encompassed the present-day 4th arrondissement. Today’s boundaries of the 4th arrondissement were established in 1860, under the Second Empire. On the edge of the 12th arrondissement, the Bassin de l’Arsenal was first an important commercial port, as barge traffic increased on the Canal Saint-Martin, until 1983, when it became a marina.