Factual error: In the final scene there is a shot which shows a Land Rover in the distance. Land Rovers were not built until after World War 2.
Factual error: The Vikings raided England in the 9th and 10th centuries. The first castles were not built until post 1066. The castle in the film has rounded towers, crenellations and machicolations, which would date it at approximately 1300. That would tend to suggest that these Vikings were 400 or 500 years old, or that they had access to a time machine.
Factual error: This horror-fantasy film (a 1958 knockoff "The Mummy") was inspired by the faceless whole-body plaster casts taken from the volcanic ash of Pompeii, at the foot of Mt. Vesuvius. The running mistake in this movie is the assumption that the faceless bodies of Pompeii were 2000-year-old mummies preserved by volcanic ash and could, thus, be re-animated (by radiation, in this case). The fact is, the victims at Pompeii were originally encased in hot volcanic pumice and ash, and the corpses then disintegrated, leaving hollow "molds" of human bodies underground. It wasn't until the mid-19th Century that archaeologists first discovered the molds, filled them with plaster, then extracted the whole-body plaster casts for display. Since the faceless bodies of Pompeii are nothing but modern plaster casts, there would be nothing to re-animate, by radiation or any other improbable means.