The usage of the term "Norman" is a huge Anachronism Stew, as it comes from the 10th century. Also, Norman chiefs being units from the Britannian civilization in the third game is strange as well, as Normandy is a northern region of France that had little interaction with Britannia at the time (the first and second games got this a bit better by having them in the Gaul civilization, but it was changed nobody knows why).
In the third game, Britannian druid Penut chants the name of Kathobodua, a continental Gaulish goddess who has never been mentioned in the comparatively richer Britannian religious sources. This is presumably an in-game reference to the goddess's role in the first game, though. Similarly, Fand is an Irish deity, not a Gaul one.
In the game, Viriathus' priestess is named Catriona. In reality, "Catriona" is a Britannian form of the Greek name Catherine, which wasn't known in the British islands until almost the Middle Ages.
The Iberian faction in the game is actually an outlandish mishmash of cultures from the Iberian Peninsule, including, but not limited to, southern Iberian Greco-Phoenician art style, northern Celtiberian architecture, and some Celtic-looking warrior units - basically a Hispanic version of Mayincatec.
In real life, Hispania was very diverse in the religious aspect too. Northern tribes apparently had diviners who were entasked with sacrifices, while southern regions could have believably shared Carthage's Phoenician pantheon and priests. The game, however, opts to portray their religious class as composed by female priestesses clearly based on the dama statues of Spanish archaeology, despite they were limited to the Mediterranean coast (and we don't even known if they are meant to represent priestesses, goddesses or noblewomen).
As with the Iberians, the Britannian civilization is a rather surprising union of tribes that in real life weren't exactly friendly to each other. They have Caledonians, Highlanders and continental units working along with warriors who look vaguely like southern Celts.
Names like "Gawain" and "Gareth" are both misplaced and anachronistic, being medieval Welsh names rather than anything a Britannian Celt could have sported.
Rather than looking Phoenician in style, the Carthaginian civilization from the game has somewhat of a Muslim touch, with Arabian-looking architecture, camel cavalry and people in turbans and djellabas, while at the same time having a lot of Sub-Saharan tribal imagery. (By the way, Carthage's religion was carried on by clean shaven priests, not by bearded "shamans", which is in fact a term for Eurasian religious specialists.) Also, some of their in-game quotes mention Moloch, a posterior and vaguely pejorative Hebrew word for any Phoenician god; a person from Carthage would have referred to his god as Baal Hammon or simply Baal.
Numidians and Mauritanians in real life were olive-skinned Berber people, while the game portrays them as being fully black and looking like tribal warriors of the Darkest Africa (though they did get the right skin tone for the Libyan lancers, even if they still got their gear and weapons wrong). It extends to the hero units too, as the game's tendency to assign them random names and character pictures messes up things: chances are that, after recruiting a Carthaginian hero, you will end up with a guy sporting a Phoenician name and a character pic that represents an "Angry Black Man" Stereotype.
The game's Egyptian civilization looks straight out of The Egyptian, with its architecture, religion, clothing, weapons, armor and the like resembling how they used to be in the Eighteenth Dynasty at the very least (that is, earlier than 1200 BC). In real life, by the time of the Roman Republic, Egypt had been Hellenized in plenty due to the conquest of Alexander the Great some centuries earlier, and all the noted aspects of the Egyptian life would have been more distinctly Greek/Macedonian than what people usually pictures when they think about ancient Egypt.
Military
Gladiators were entertainers and prizefighters, not soldiers. Rome would have never sent them to the battlefield, at least not while wearing gladiator gear, which was designed only for the spectacle and was quite impractical in serious warfare. Speaking about it, the game mixes up two kinds of gladiators in their portrayal: their unit wears a murmillo helmet yet wields a retiarious trident.
There really were foreign slaves who gained their freedom and became soldiers and officers in the Roman army, just like the game's liberati, but, upon doing so, they would naturally adopt the Roman army's attire and weapons. In the game, they are portrayed as barbarian-looking guys wearing loincloths and wielding axes.
Tribunes were military commanders, not frontline warriors, and they definitely didn't fight while Dual Wielding gladii.
Celtic and Germanic warrior women are really recorded in history, but their portrayal in the games is strongly rooted in either Rule of Cool and/or Politically Correct History. In real life, while it was relatively frequent for women from these cultures to accompany the men to the battlefield, they usually took support roles, acting as cheerleaders and combat medics, and would only join the fight if the men were overpowered (or if they tried to desert, in whose case the women would attack them). Conversely, the game has them forming specialized, Amazon Brigade-style units, which was historically anecdotical and only found in some late Gothic tribes.
In opposition to the previous, warrior women are well attested in sources about ancient Hispania, where they appear as defenders of cities, but in-game Iberians are ironically the only barbarian faction that does not have female fighters (though Imperivm II compensates this by having some Gaul warrior women among the Iberian forces in campaign, probably in reference to the mentioned records).
Britons and Gauls were famous for using war chariots, but none of those civilizations has them in the game. Meanwhile Rome, who did not use them for warfare, does have.
Both Britons and Iberians have archers in the game, when in real life there is little evidence of bows being used in any (there is some in Spain, but it comes from Phoenician and Greek colonies, where usage of this weapon was clearly imported and limited to those cultures). In turn, the sling was a popular weapon in Britannia, yet they don't have any in the game.
Gauls definitely did not have a caste of sacred warriors wielding tridents, nor used giant double-headed battle axes (they might have had a minor version which was actually a throwing axe).
Germans did use battle axes and clubs, but certainly not the exaggerated giant halberds and spiked maces they wield in the game. Also, they rarely used helmets, most of the time being only the leaders who wore them, while their in-game civilization has an impressive variety of helmets and headgear.
Just like their civilization altogether, the games's Iberian army combines Mediterranean coastal elements (falcatas, oblong shields, ornated helmets) with Celtiberian ones (white tunics, oval shields, crested helmets), along with Cantabrian (battle axes), Lusitanian (caetras, leather caps, guerrilla fighters) and downright fantastical (the spear weapon wielded by said guerrilla fighters). Also, like Britannia, they feature archers, even although cultures from Hispania used javelins and not bows.
Related to the previous, though so pervasive in media that it could be its own trope in the vein of Every Japanese Sword is a Katana, the usage of falcatas by Viriathus and the Iberian faction in the game is inaccurate. As said above, the falcata was limited to the Mediterranean coast of Iberia and was rare to find on the rest of the peninsula, where straight swords were much more popular. While Viriathus and the rest of the Lusitanian elite might have probably owned falcatas thanks to trading and plundering of the southern territories, they probably didn't have it as their standard weapon.
The portrayal of Carthage's army in the games is spectacularly wrong. To list it, it lacks any cavalry (in real life, they were known and feared by their varied contingents of horsemen), have camel riders (which they never used in real life), employ Numidians as footsoldiers (Numidians were historically renowned riders who were barely used as infantry at all), portray Libian as unarmored javelin throwers (in real life, they were heavy infantry in the vein of the game's noblemen) and have unarmored Mauritanian guys Dual Wielding sabers (which is entirely fantastic).
Carthage's Sacred Band was actually composed by spearmen in the phalanx style, not swordmen as portrayed here (they did carry swords, but just as a secondary weapon). Also, its presence in the game's events is an Anachronism Stew: while Carthaginian armies did feature commanders and small forces recruited among Punic citizens, the real Sacred Band was disbanded sixty years before the very First Punic War and its unit name was never used again.
In the game, Carthage's specialty as a faction is playing a Galactic Empire-style human wave strategy, with some units even being explicitly designed to inflict damage by dying. This kind of warfare was not unknown to ancient Carthage, as the Sicilian Wars sometimes did see Carthaginian generals trusting more on their mercenaries' numbers and aggression over real military skill, but it had been phased out by the time of the Second Punic War, when the heavy indemnizations to Rome and the influence of Hamilcar Barca's leadership had pushed them into adopting a more conservative, cerebral approach to warfare. By that point, this strategy would make sense if employed by Gauls, who really loved the good ol' frontal charge and considered honorable a stupid death, and whose high number of tribes opposed to Rome made it easy to form large amounts of manpower.
Most of the Egyptian civilization's named warrior classes (Horus warrior, Anubis warriors, Nile guards) are fictional, as well as some of their weapons and armor. Also, some of their warriors are unarmed and barefoot, fighting in only loincloths, which is inaccurate, as warfare was one of the few fields where they bothered to wear more protection. In any case, nothing of this makes sense historically because, as mentioned above, Egypt by this point had long abandoned those ancient aesthetics and adopted the Hellenistic model of warfare.
Characters
The first game has a prologue set in 132 BC, in which the very first words of the narration claim that Augustus Caesar currently has a firm hold on Rome as the first Emperor. This is off by more than a hundred years: Julius Caesar (let alone his adoptive son Augustus) hadn't even been born yet. This is not hard to notice if you're aware of the widely known fact that Julius Caesar was assassinated in 44 BC.
While it can be forgiven if we interpret that the entire Imperivm II tutorial level is meant to be non-canonical fantasy, it still contains a strange piece of history where Viriathus is somehow a Celtiberian (there are some fringe theories about it in real life, but mainstream historians are unanimous that he was a Lusitanian), learns under Caraunius (who in real life was probably younger than Viriathus) and a Gaul druid (and one with the Germanic-sounding name Haaser), and eventually becomes the chieftain of Lutia (a city that was never allied to either Viriathus or the Celtiberian resistance). He is even given the unit skin of Larax from the first game, who is clearly Gaul in appearance. For extra bizarreness, there is a patch of desert and a Carthaginian settlement in the same map, and the English version of the game changes the locations to Kebatha (a Gaul village from the first game) and a fictional city named Barezia.
The Romans from II apparently invented Time Travel, because their Senate in 264 BC somehow features Cato the Elder (234-149 BC) and Cicero (106-43 BC), at the same time nothing less. The former even says his classic quote "Carthage must be destroyed!" despite the fact that the real reason that led him to utter it in real life (namely, that Carthage hadn't been completely destroyed and was slowly rebuilding its power) didn't even exist at the time (Rome and Carthage hadn't really clashed yet).
At the campaign in Zama, Scipio Africanus is portrayed as an old gentleman, recycling the character picture of Canon Foreigner Senator Anteros from the second game. In real life, Scipio was actually 34 when he fought that battle.
During the Boudica campaign in Imperivm III, her husband Prasutagus is not mentioned, making it look like Boudica had always been the sole governor of the Iceni. This version of Boudica also revolts only due to Rome's abuses and high taxes, when in real life she had a much more personal reason, namely that she was flogged and her daughters raped by Roman centurions after Prasutagus' death. Finally, the game follows the pop culture trend of portraying her as an Action Girl, even giving out a picture of her fighting literally at the front lines, which no historical source backs up.
Events and battles
In the first game, the Battle of Gergovia is portrayed as a massive, Napoleonic-style open field battle in a plain. This was fortunately corrected in the third game, where it is accurately portrayed as a mountainous assault, as it was in real life.
In Imperivm II, Numantia is placed in midst of a plain field, when the reason of its legendary resistance in real life was precisely its emplacement in a mountainous terrain with many natural defenses. Again, the third game corrected it.
The second game has the Senate receiving the news about Sicily and deciding to send Appius Claudius Caudex to intervene against Carthage, thus giving the impression that the Senators were the masterminds behind the move. In real life, most of the Senate was actually opposed to the intervention, and it was Appius himself who forced them to allow it by appealing to the citizens.
Gades (which at the time was actually named Gadir) was a coastal city on a narrow slice of land, not a fortified city deep into land as it was portrayed in II. Also, in the game Gades is an Iberian city the player has to conquer, while in real life it was a fellow Phoenician colony whom Carthage had as its ally.
Following the previous point, II gives the impression that Hispania was a completely unexplored, savage land until Hamilcar's arrival, and implies Romans only came to the peninsula chasing the Barcids. In reality, Carthage had been formally trading with Hispanic tribes since several centuries earlier, which is why Hamilcar already knew exactly where and who he should attack to make a few good conquests (unlike the game, in which he's pretty on the dark until he captures Gades). Similarly, Rome had its own field of influence on Hispania before the Second Punic War, and it was precisely because Hannibal attacked their territory that the war exploded.
Although the game doesn't mention it by name, the Roman mission in Germany from Imperivm III is clearly meant to be the Battle of the Teutoburgo Forest, where Varus was defeated by Arminius. However, the events and disposition of the battle are completely divergent from real history: in real life, Arminius attracted Varus out of his camp through deception (Varus believed they were allies by this point) and led him to a large ambush in the forests, while in the game, Arminius is ambushing just a small force of a city ruled by Varus which he has to capture.
Viriathus' headquarters are located in Mons Herminius in his campaign, probably because it was, for a long time, believed to be his place of birth. However, this theory was already Dated History by the time the game was produced. As we don't know Viriathus' birthplace, it would have been more accurate to locate his fortress in Mons Veneris, which sources name many times as his main stronghold.
