The Seine fed the ditches encircling the medieval Bastille.
The fortress was built on Paris’s eastern side, where defenses were needed against attacks approaching from the direction of the Porte Saint-Antoine. Its surrounding ditches were not merely dry trenches: they were supplied by water from the Seine, adding another obstacle before attackers could reach the stone walls and towers.
The Bastille developed from earlier defenses associated with the city walls. Charles V ordered a much larger fortress on the site in the late fourteenth century, and construction began in 1370. The completed structure had eight towers and thick connecting walls, with the water-filled ditches reinforcing its defensive design.
The river detail is sometimes confused with the Marne because both rivers are associated with the Paris region. The Marne does eventually join the Seine east of Paris, but the Bastille’s ditches are specifically described as being fed by the Seine. This water system belonged to the medieval fortress’s defensive setting; by the time of the French Revolution, the Bastille was chiefly remembered as a royal prison and political symbol rather than as an active frontier fortification.