Rollo was based on a real Viking called Hrolf (latinized as Rollo) who went on to conquer parts of France. Those parts are now known as Normandy: "Land of the Norsemen". Hrolf was the ancestor of William the Conqueror. Actually being William's Great-Great-Great Grandfather.
The detail that the Norsemen were tattooed is not necessarily speculation. Eyewitness accounts from the Viking Age report that Vikings were, indeed, tattooed. This was also a feature of Celts and Slavs, supporting the idea that the practice was widespread in Pagan Europe.
Gustaf Skarsgård assisted some of the other actors in the pronunciation of certain Scandinavian words. According to him the word the English speaking actors found most difficult was the name "Knut", which is often anglicized as "Canute" in history books and fiction. In Swedish and Norwegian Knut sounds like "Knuut".
The women in the opening credits are supposed to be the 9 daughters of Rán, the personifications of the waves. The title sequence is supposed to show the separation of a viking from the living, with the ornaments of his life (gold and weapons) floating down around him. During the shot of the rolling thunder there is a single frame showing Hel Lokisdottir: the goddess of death.
Ravens appear prominently in the series. Ravens had great cultural and religious importance in ancient Norse society, as they were seen as the agents of Odin who was said to own two ravens: Hugin (meaning "Thought") and Munin (Memory) who flew around and gathered information for him. One of Odin's many nicknames is Hrafnaguð, which means "Raven God". Vikings would carry banners depicting ravens. Ragnar's forces carry such banners. As such, the pagan characters in the series are likely to believe that when ravens appear, they are watched by Odin.
It is speculated that instead of making the vikings in the series one nationality, the producers chose to make them a collected "Scandinavian" people. Thus, the locations representing Kattegat strongly resembles Norwegian landscapes while the plots are based on Icelandic sagas about Danish heroes, kings and battles, and Ragnar Lothbrok and his sons who supposedly were Swedish. The idea is also supported by the name of the settlement "Kattegat" - in real life, this is not an area, but the sea connecting Denmark, Sweden and Norway.
The first season has nine episodes. 9 is a very special number in Norse Mythology: Odin hung upon Yggdrasil for nine nights to learn the secret of the runes, there are nine worlds in the world tree, Heimdall has nine mothers, Odin owns a ring which creates nine new rings every night, Ran has nine daughters embodying the waves of the sea, there are 9 great lindworms (dragon-like serpents), every ninth year nine males of every species is sacrificed to the gods, Freyr had to wait nine nights until he could marry Gerd, the valknut symbol has nine points, Thor will walk nine steps before dying after killing Jörmungandr, Odin broke free and killed King Geirröd on the ninth night of his captivity and Odin knows 18 (9x2) rune charms.
There is almost no evidence of what haircut the Vikings wore other than beards for the men and ponytails for the women (based on contemporary Norse art). Therefore the haircuts of several of the Vikings were designed so that hair would not stick out beneath a helmet so enemies would not be able to grab a hold of their hair from behind.
"Loðbrók" is not a family name but a nickname meaning "hairy-breeches", presumably from a habit of wearing fur leggings. Vikings often acquired colorful nicknames derived from personal habits, characteristics or traits. The sagas and histories of Vikings are populated with such figures as Helgi the Lean, Ketil Trout and King Harald Finehair (formerly Harald Shag-hair).
Jarl Borg was originally a Swedish jarl and Horik would send Ragnar to Sweden to settle the dispute. When Gustaf Skarsgård read the script he wrote to Michael Hirst and informed him that 1) "Sweden" did not exist during that time and 2) Uppsala would most certainly be part of it. Hirst changed it to Götaland (Land of the Geats). Sweden was formed as a consolidation of Götaland and Svealand (Land of the Swedes) where Uppsala is located. While it's very uncertain on what exact date "Sweden" was formed (Götaland and Svealand ruled under a single king is commonly set as a point) the term would not have been used in the time of the Vikings.
The show takes its premise and basic plot from the Icelandic sagas Ragnars saga loðbrókar (The Saga of Ragnar Lodbrok) and Ragnarssona þáttr (Tale of Ragnar's Sons). Some have suggested that the actual protagonist of the cycle is Aslaug. Lagertha does not appear at all in the sagas. She appears in Saxo Grammaticus Gesta Danorum (which is also the basis of Hamlet). Most scholars agree that while there is some historical basis for Ragnar, the father of Björn Ironside and Ivar the Boneless and the plunderer of Paris, Lagertha seems to be entirely made up by Saxo and drawn upon various mythological characters, amazons and shieldmaidens.
The nickname "Loðbrók" refers to a saga about Ragnar that Saxo wrote. In the story, Ragnar dresses himself in a fur coat and fur pants and walks through icy water in order to get the fur frozen. This is to be shielded from two Lindorm (mythical serpents) that have trapped a maiden in her house. Afterwards, the maiden's father mocks Ragnar for having "fuzzy pants", i.e., "Loðbrók".
When Ragnar explains his plan to reinvade Paris, he says they need to protect their heads, and he takes a horn that he was holding and puts it up against his head to look like the stereotypical Viking headwear found in re-imagined depictions of Norse warriors by Victorian artists of the 19th century and pop-culture. There is no historical evidence that Norse warriors ever wore horned helmets.
The symbol on the shields used by the warriors of Hedeby are two "Fe"-runes - a letter used by Germanic peoples in the dark ages equal to the Latin letter "F" - laid back to back to look like a tree. This is a highly symbolic design as carving several runes into another, a "bindrune", was supposed to summon the magic of the rune. In this case "Fe" symbolizes wealth.
The soldiers of Wessex are wearing helmets of a period 700 years later than depicted in the story. These are burgonets of an Italian style, more at home in Renaissance Florence than England. Moreover, the kite shield was not introduced into England until c.1000, just before the Norman invasion. Other English troops are seen wearing padded leather jerkins of 1200/1300s, so are also completely out of period. The Swedes are seen wearing helmets used by Anglo-Saxons in the 8th century, not by Swedes.