You're right about the crossbows, but the incendiary weapons would not have been unfamiliar to the Franks. Such weapons were used as far back as the Assyrians in 9th cent. B.C. They can be based on petroleum, sulfur, other chemicals. About 200 years before the Siege of Paris, "Greek Fire" was used in the Siege of…

The historical Vikings did transport ships over land, especially moving from fjord to fjord. The ships were actually quite light in weight. At sea they had to load heavy ballast rocks to make them seaworthy. But you're right in asking whether there might have been a flatter shore to do this; maybe the land distance…

The river may have been cleaner in those days. Are the Vikings downstream from Paris?

The actress who played Yidu said in an interview that the original script was to have her in 8 episodes and be killed during the raid by the Franks on the Viking camp as she defended Ragnar's boys. This revised demise by sudden drowning makes her character seem pointless except as the means by which Ragnar can express…

The woman was one of Lagertha's battle maidens, probably the one whom L. carried off the battlefield wounded.

Perhaps her identity as the emperor's daughter will play a role here. What are her intentions? Is she striving to lead the Vikings to destruction by addicting their leader? Perhaps she is taking revenge for being mistreated as a slave. Or is she trying to get back to Paris, maybe to take revenge there on the Franks…