The three Viking ships in the film were designed using blueprints for an actual Viking ship salvaged from the water and restored by a Viking museum in Norway. It turned out that the boats built for the film were too accurate, because the modern actors were taller than their historical counterparts. Every other oar hole had to be plugged so the modern men would have room to row with a full oar stroke. Otherwise, they would hit the backs of the oarsmen seated in front of them when pushing the oar handles forward to start each new stroke.
Stuntmen had practiced for weeks for the oar walking scenes. Kirk Douglas told director Richard Fleischer that he could do it and did several times. At one point when he did fall in the icy water he calmly swam over to the camera boat and asked if they had gotten good shots. He then swam back to the Viking longboat. Fleischer noted they were watching and filming an activity that had not been done in 1000 years.
At the end of the film, a Viking ship is set afire by flaming arrows in a rendering of a traditional Viking funeral. Director Richard Fleischer took great care to have the archers practice the moment, training them to release the arrows on the count of "three," and hoping at least some of the arrows would arc properly to hit the sail of the ship and set it on fire. When the time came for the live shot, the director only reached the count of "two", when one over-eager archer loosed his arrow. As luck would have it, the arrow arced perfectly and hit the sail. Then, Fleischer called, "Three!" and the other archers loosed their arrows. Fleischer decided that he liked the one, single arrow being launched first, and kept the shot in the film because it looked like part of the ceremony.
The movie was shot in Kvinnherad, Norway, which is a small community by the Hardanger Fjord. Many locals still remember "when Hollywood showed up in Kvinnherad", and many of the extras are local inhabitants.
Director Richard Fleischer spent two years researching the Norse civilization in preparation for doing this movie. This included the actual designs for the Viking ships they used and the breed of horse that they rode.
In an interview given shortly after the release of the film both Kirk Douglas and Tony Curtis said that they had been cold during the entire filming. The water in the fjord was just above freezing and the air temperature was only slightly warmer.
There was actually a kingdom called Northumbria, whose history undoubtedly formed part of the basis for the novel from which the movie was made: It was formed about AD 616 by the union of two smaller kingdoms, Bernicia and Deira, with Edwin, son of the Deirian ruler Aella, as its first king. In AD 867 it came under Viking control when conquered by two brothers, Ivar and Halfdan Ragnarrson, and integrated into Danelaw, a Viking kingdom already established in Britain. The now-earldom of Northumbria was later divided between two rising kingdoms, Scotland and England. The English portion became known as "Northumberland."
The story has it that King Ælla killed the father of Ivar, Halfdan and Ubbe Ragnarsson by throwing him into a snake pit. Halfdan and Udde tried to avenge their father but were beaten by Ælla. In a second attack by the two brothers, some time later, Ælla was captured. It was Ivar who suggested that Ælla be killed by having the bloody eagle carved on his back.
Althrough the plot is mostly historical fiction, there really was a king in Northumbira called Aella who usurped the throne and was soon disposed of by invading Vikings. It's worth it to note that the puppet king the Vikings installed was called Egbert, which of course is the name of the English collaborator in this film.
At the final battle Einar has lost an eye and Eric a hand. In Norse mythology it's said that Odin is one-eyed and Týr one-handed, hence Odin is called "One-Eye" and Týr is refereed to as the "one-handed god". Odin and Týr are the gods of war.