Excerpts from Books and Wikipedia
"The feeble and crumbling Carolingian Empire was dealt a heavy psychological blow in March 845 when one hundred Viking ships managed to sail down the Seine, causing death and destruction as they went. . . . Once in Paris, the Viking raiders plundered the city and many churches were either desecrated or destroyed. The ordeal for the people of the city was finally ended when Charles offered a huge ransom. . . . For the ailing Carolingian Empire, the most vulnerable areas were in the Low Countries and the regions of Gaul and Germania, where many navigable rivers gave the raiders easy access. Paris was attacked three times in the 860s, the raiders leaving only when their haul of loot or bribes were considered large enough. Facing almost annual raids, Charles the Bald at last took positive steps to counter the threat. In 864, the Edict of Pistres brought about a development that included the creation of a large force of cavalry and a series of bridges across the Seine that Viking longboats would not be able to pass. These bridges would prove highly effective when the Vikings besieged Paris again in 885." [1001 Days]