All of the horse riders are shown riding with stirrups. The stirrup did not come into use in Europe until the 6th century, so Romans and Britons riding horses would not had stirrups at the time depicted in the film, the 2nd century AD,
Quintus Dias muses that he does not know if Arianne is an angel or devil. Unless Quintus was Christian, Jewish, or Zoroastrian, he would not have used this concept at the time of this movie's events in the 1st century AD, when Christianity had not widely expanded in Europe.
There are decapitations aplenty in this movie, with many lightweight weapons, short swords and small axes. Yet not even executioners during medieval times were always able to chop off a head with one stroke, even though the criminal was resting his neck on a chopping block and holding perfectly still and the executioner using a very heavy axe.
Quintus Dias is not a possible roman Name. Quintus would be a possible pronomen, but there's missing the nomen gentile (family name, ever with an "-us"-Ending), every Roman had. Dias would be a strange cognomen (third part of roman names). Example: Caius Iulius Caesar = Quintus XXXXX Dias.
The spear the IX Legion carries is a period-correct piece of Roman military equipment, known as the hasta. While certainly less well-known than the pilum (plural pila) throwing spear, the hasta would be a better tactical choice for a legion marching into heavy forest where the space required to bring up and throw a pilum properly is not guaranteed. The hasta, being basically a "typical" spear, is a more versatile weapon. It should be noted that every legionary was issued both a pilum and a hasta, though the pilum featured more prominently in the Romans' preferred open-field strategies.