Though the name of the city has not changed much since the 11th century, we note that the word “arche” was originally plural.
What accounts for that is that “arche” did not refer to the arches under the bridge but to citadels such as Arques Castle near Dieppe. As a matter of fact, with the “fort of Limaie” on the right bank, Pont-de-l’Arche constituted a second fortification for the city was surrounded by walls.
It’s quite logical then that the people in the area pointed out its main feature to call the place; a bridge defended by two forts, two arches (which must have been pronounced in the Norman way “arques”).
Later, the city prevailing, only one fort was retained, henceforth Pont-de-l’Arche in the singular. Otherwise, how could one justify calling the bridge after only one of its arches?