.. [their] art is distinguished for its extensive curves and intricate knot work which is used to form complex decorations for weapons, jewelry and body tattooing." (Crystalinks) it seems that Guinevere is actually wearing a good deal more than one would expect from a Celtic warrior, and her knotty outfit is fitting. However, critics are fair in complaining that she might perhaps be wearing a bit too little for the weather. Speaking of minor inconsistencies, it seems a bit odd that Guinevere's broken hands heal almost overnight - and subsequently don't interfere with her shooting.
Guinevere's clothing and hands are not the only thing critics point to; they also suggest that she is being presented in a historically inaccurate way as a female warrior. In many movies it may be true that women are ahistorically buffed up - however, this is not necessarily one of those cases. While the historical Gwenhyvar is unlikely to have been a warrior, there is no historical reason to suggest that she was not a warrior. Celtic women were considered equal with the men, and many of them went into battle. An article published in Keltria: Journal of Druidism and Celtic Magic, explains how primary sources indicate that "Celtic women also took an energetic part in melee. It was said that a Celtic man with his wife could hold off an entire troop of Roman soldiers... On the whole, these female champions are described as both beautiful and courageous....Sometimes referred to as Ban-faith or 'prophetess' they were experts in divination and supernatural wisdom." (Hansen)
That Guinevere might be such a warrior-prophetess would explain her close relationship with Merlin, and also the meaning of her Celtic name Gwenhyvar (which, as mentioned above, labels her as an enchantress). Celtic women were not only active in military arts, they were leaders in society and frequently considered to be in charge of the initiation and training of male warriors. This historical element may contribute to the way in which Guinevere in this story selects Arthur and prepares him for battle.
Celtic culture was matrilineal, which is to say that inheritance and power is determined not in the passage from father to son, but from mother to child. Most great warriors were training in their craft by women, and one assumes that if Arthur had lived among the Celts as a young child and come to military age among them, he no doubt would have been trained by a woman - particularly in a mythical sense, as he was destined to be a great hero. Another great mythical hero of that era, Cu Chulain, sought out his female war-teacher across the water on an island of shadows (not unlike Arthur's quest to find Guinevere in old Welsh texts). This warrior queen "was reputed to be the matron of self-defense and female independence as well as the guardian of young people who seek to know their full potential. Men came from further afield than Gaul to train with. her, and if they passed her rigorous tests they were more feared than any other fighting men." (Fox) Cthulain was then trained by her to be the strongest warrior in land. Arthur may be related to Chulain in terms of mythological development or even historically, as many of the same stories are associated with them. The warrior-queen who trained Cthulain was also Ban-faith, an enchantress, and this creates a new sense of why the 2004 movie portrays Guinevere as a warrior. The transformation of Guinevere becomes appropriate in this sense - that because her name indicated her being an enchantress in the original texts, and because there is a strong historical link between female warriors and enchantresses, she becomes a warrior in this version and responsible for Arthur's rise to power.
So far, it has become apparent that Bruckheimer's Guinevere is not historically accurate at all in terms of linear, historical facts. She is not, one might say, biographical. However, it is also relatively clear that this is the fault of the vagaries of the King Arthur myth (namely, that it is impossible to pin down historically) and of the overall non-historical plot-line which has battles happening where there were none, Romans still installed in all their glory on the Hadrian wall decades after they'd left, and heretics preaching things that never occurred to them. The actual characterization of Guinevere (other than this anglicization of the Celtic name) and her relationship to Arthur is not necessarily based on historical fact or legend, but it is an entirely legitimate as speculative historical fiction. It is entirely possible, though not traditionally assumed, that Guinevere rivaled Arthur in social and military power. It is even a reasonable theory, based on the culture's matrilineal approach heredity and power and the prominence of women warriors and religious leaders. Guinevere in this story is certainly more realistic than in the majority of Arthurian stories which take a medieval approach to gender issues and assume that she was forced into a relationship with Arthur (unlikely in a culture where women had full rights and were the carriers of inheritance), that her love affair with Lancelot couldn't be resolved because Arthur would be legally obliged to have them both executed (death as a penalty for adultery would have been unheard of in a culture where women were not property to any degree), or suggest that she was an entirely passive figure. If there was a historical Celtic Guinevere, it is likely that she was a relatively strong figure, loyal to her husband, and both intelligent and spiritually powerful. It is even entirely feasible that she was a warrior and fought beside him. Considering that Guinevere is never mentioned in early texts which support theories that Arthur was of Roman extraction and culture (she is only evident in the Welsh texts, which portray both her and Arthur as Celtic), it seems unlikely that she was a passive member of the patriarchy. In this respect, at least, the movie is historically accurate - and this truth has been so long overdue for credit on big screen that the timeline inaccuracies may be easily forgiven.
Britannia. "King Arthur: What do Modern Historians Think of Him?" Britannia. available at http://www.britannia.com/history/historan.html
Crystalinks. "The Celts" Crystalinks. [online mythology database] available at http://www.crystalinks.com/celts.html
Davey, John. "Celtic Britain: Now Just Who Was Arthur?" available at http://www.kessler-web.co.uk/History/FeaturesBritain/BritishArthurWho.htm
Ford, David Nasj. "Early References to a real King Arthur." Early British Kingdoms. available at http://www.earlybritishkingdoms.com/arthur/karef.html
Fox, Jo. "Women of the Celts in Myth, Legend, and Story." SkyeViews Issue 8 June 1996. archived at: http://www.pabay.org/skyeviews.html
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Green, Thomas. "The Historicity and Historicisation of Arthur" Arthuriana. Oxford, 1998. available at http://www.arthuriana.co.uk/historicity/arthur.htm
Hansen, Daniel. "Warrior Women" Keltria: Journal of Druidism and Celtic Magic. Issue 34 Summer 1995. archived online at http://www.keltria.org/journal/warrior.htm
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Scheib, Richard. "King Arthur (2004)" Fantasy Reviews. available at http://www.moria.co.nz/fantasy/kingarthur04.htm