The number of episodes in the first season is 9. 9 is a special number in Norse Mythology. There are 9 worlds in the world tree Yggdrasil (that is spelt with 9 letters in modern English), Odin hung himself from Yggdrasil during 9 nights, Odin knows 18 charms (9x2), Hel was given command over 9 worlds, at Ragnarök Thor will take 9 steps after killing Jörmungandr and die from it's venom, Hermondr rides 9 nights to reach Helheim and free Baldr and Ægir has 9 daughters.
The music that plays during the raid in episode 4 is a "Fehu" by Wardruna, and it's played with the kind of instruments the vikings actually used. The lyrics are in Old Norse and based on Norse and Anglo-Saxon rune poems: "Fe [wealth] causes strife amongst friends, The wolf feeds in the forest, Fe is joy to man, strife amongt kin, path of the serpent, The snake lies coiled, Hidden, it waits beneath, like a frost-covered field, Strife that kinsmen suffer." Ironically, all the parts criticizing wealth are from Norse poems, while "Fe is a joy to man" is from Anglo-Saxon poems.
King Ecbert and Prince Aethelwulf of Wessex appear to be good and just monarchs whose subjects are treated well; indeed, it almost makes the Vikings Villain Protagonist (particularly King Horik). This would kind of make sense in a mythological way — Aethelwulf's son, Alfred, was the only English King to ever be called "The Great," and he was a fair and just ruler with quite a bit of strategic and tactical nous as well.
Think about the crucifixion scene from Athelstan's point of view for the full Fridge Horror. He's going to die in hideous pain, surrounded by enemies, it could take days, and then what? No heaven for him, because he's an apostate and a killer. No Valhalla, because he screamed in agony and begged for his life, not a brave death in battle. Whichever religion is right, he's getting the bad end of the deal... at least until King Ecbert came along and bought him more time.