Orestes, who was partly of German blood, was historically the magister militum — the senior officer of the Roman army second to the emperor. He had indeed promised his German foederati a third of Italy to settle in but not to Odoacer personally. Orestes was himself an usurper, having used his power over the foederati to depose the legitimate emperor Julius Nepos and inaugurate his son Romulus Augustus.
Romulus's coronation and capture is depicted as taking place in Rome, whereas his historical reign and abdication took place in Ravenna.
There was civil strife in the Eastern Roman Empire at the time but the Emperor alluded to would probably have been Zeno (explicit in the novel). The Eastern Emperor eventually neutralised Odoacer when the latter became too popular with the people of Italy.
The Emperor Tiberius is referred to as "the last of Julius Caesar's line". Inconsistently, Romulus Augustus is also depicted as the last of Julius Caesar's family line and Tiberius as one of his ancestors. Tiberius was actually the second of the five Julio-Claudian emperors, succeeding to the role of emperor as the adopted son of Augustus, who had himself been adopted by Julius Caesar. Augustus had named several of his own relatives as heirs, but as they all died, he named a son of his second wife, Livia, by her first marriage. Thus, Tiberius was not the last of any line established by Julius Caesar (Augustus' maternal great-uncle by blood) as he had no blood relation to Caesar at all. His successors, Caligula and Claudius were blood relatives of Julius Caesar through a maternal blood line dating back to Augustus's older sister, Octavia. Nero, a direct descendant of Augustus, ruled roughly twenty years after the death of Tiberius and was the final emperor of the Julio-Claudian Dynasty, thus making him the last ruler descended from Julius Caesar's family - the Julii. In the film it is implied that Romulus' descent from Caesar's family is through his (unnamed) mother (which is presumably why Orestes cannot be Emperor himself). Historically, nothing is known about Romulus's mother, but the Julii had been extinct for more than 400 years before Romulus was born. He could only be claimed as "the last of Julius Caesar's line" if "line of office" was meant, as he was the last Western Emperor to bear the name "Caesar", which all Emperors subsequent to Nero adopted as part of their imperial title.
The film uses the premise of the missing Ninth Legion still existing somewhere in Britain at this time, a once popular idea among British Historians. The Ninth Legion disappears from Roman records from about 120 AD, by which time, it has been suggested, it was no longer serving in Britain. Rather, it has been thought that the Legion was destroyed in some unknown war near the Danube frontier or perhaps whilst serving on the Eastern front fighting the Parthians, during or before the reign of Marcus Aurelius. Recently, however, academic opinion has swung back to the 'destroyed in Britain' hypothesis, Dr Miles Russell of Bournemouth University demonstrating that the weight of historic evidence suggests that significant numbers of Roman troops were wiped out in the province during the early years of the emperor Hadrian. Even if, as now seems likely, the Ninth had gone missing in the province, it is highly unlikely to have reappeared then continued in existence as a military force for the three centuries between AD 117 and 460. At any rate by the late fifth century AD Britain had already long been abandoned by Rome, mobile units having departed for the continent in AD 407, more static frontier units devolving into little more than a local militia.
The uniforms and weapons of the Ninth Legion as depicted in the film (badly) reflect an earlier period of Roman history. By the time of the official fall of the Western Empire, Roman legionaries did not wear scarlet cloaks, nor did they carry the semi-circular shields of ancient times, and the short stabbing sword known as the gladius had been replaced.
Mira's weapon, the Katar or Katara (कटार), had not been invented at the time in which the action is set. Katars came into use more than 1000 years later.
A few of the castles and fortified cities in the film have round spires with pointed coned roofs, when in fact this style of buildings was not common until the late Middle Ages.
There is no record of black people serving in the Praetorian Gruard, Nor was there Indian women in the army of the Eastern empire