There actually was a regiment of Sarmataria Calvary in Britain. It was called Ala Sarmatarum or cuneus Sarmatarum. ALA means auxiliary cavalry regiment. CUNEUS means a Calvary regiment of the Numeri. Numeri were regiments first raised early in the second century from various tribes inside and outside the the Empire and various specialist regiments. A full cavalry regiment consisted of 5 turmae with 32-70 cavalrymen. By the late 4th century and early 5th century these regiments would have been very under manned. As replacements from their home lands would have been sporadic or non existent. If Arthur was a Roman he probably command a small turmae. I don't think he held a maximum rank of prefecture (read regimental commander). He would not be a Centurion which usually was a commissioned infantryman. What Arthur WAS was a most likely a Decurion a commissioned officer in charge of a turma of Cavalry. One turma would have been posted per major "fort" along Hadrian's wall to sally against raiders.
   Knights in the Roman army were noble men and officers so the movie is taking  liberties calling non-roman citizen auxiliary cavalryman a "knight". So knight was acceptable for Arthur only. All of the "knights" except Lancelot more likely held the rank of a NCO called a Sesquiplicarious. Lancelot was most likely the senior NCO called a Signifer.
   Armor the movie showed a variety of armor for the "knight" ranging from Arthur's roman to Mongol & Rus steppes. This would make sense for drafted horsemen from Sarmataria which would have included horseman of Rus and Mongol blood.
Weapons: The ice lake battle included the use of Yak horn compound bows (read Mongol) by the Sarmataria which had a longer range than Celt or Saxon bows. The glue that was used to hold the pieces of yak horn together was secret of the Mongols passed down from father to son. So with the death of the "Mongol" knight the secret would not have spread in Britain.
   Celt bows do NOT appear to be long bows. However bows as weapons would have been prolific in that they were cheap to make. The employment of Celt archers in the last battle made sense. The Romans used archers as auxiliary support troops NOT main battle troops.
   Crossbows: No way the Saxons used these. To be powerful enough to penetrate armor a METAL bow would have to be employed. That technology did not come to use for another 700 or so years. They would have employed bows similar to the Celt bows.
   Roman artillery used in last battle was NOT appropriate. What I saw were miniature Trebuchets. Again that counter weight technology was not available for another 700 or so years. What WOULD have been appropriate would be the use of Ballista. It looked like a large crossbow that shot arrows the size of your arm or stones OR crude catapults. The energy for these resided in some sort of composite animal sinew and resin. Again only a Magister ballistariorum (NCO of Artillery) would have known the secret of the composite. Nowhere apparently was it ever written down. It is unknown today.

Respectfully submitted,
Captain of militia (retired)
Thomas D. Pufnock