This is what Wikipedia has to say:
Late in the 3rd century AD, the invasion of Germanic tribes, beginning with the Alamans in 275 AD, caused many of the residents of the left bank to leave that part of the city and move to the safety of the Île de la Cité. Many of the monuments on the left bank were abandoned, and the stones used to build a wall around the Île de la Cité, the first city wall of Paris.
And then, 200 years later:
In 461, the city was threatened again by the Salian Franks, led by Childeric I (436-481). The siege of the city lasted ten years. Once again Geneviève organized the defense. She rescued the city by bringing wheat to the hungry city from Brie and Champagne on a flotilla of eleven barges.
Okay, so, even if Paris still consisted only of the walled Cité and the population was fairly small (a few hundred people, perhaps?), traveling to Champagne and then coming back with eleven (!!) barges is still a pretty monumental task. How can you sneak that many barges in without being seen? How can you sneak them out again? Or did Childeric allow it? Take a look at this picture:
La Cite is the larger island. The smaller one behind it is the Island of St. Louis. In order to travel to Champagne by water, you'd have o head south (towards us). With Childeric's troops stationed on both sides of the river, and his boats patrolling the waters day and night, Genevieve seems to have made repeated trips out of the city, and then back in - how?
Apart from that, what was so important about Paris (such as it was in those days) that the siege went on for ten years?
Something's wrong with the picture here. What am I missing?