The thing is that this is just wrong. Bannerlord doesn't have "little resemblance to history". It is literally real history/life copied directly and then with the names, timeline, and events shifted around a little.
The Roman Empire began as a tribe who overthrew a kingdom and founded their own city with the legendary founder Romulus. This tribe became a republic with tribunes, but due to decline of republican values eventually became an empire, that conquered the Celts and spanned almost the entirety of the continent barring the far north, and also spread to the shores of the southern sea. Eventually, having difficulty maintaining their empire, they had to hire out recently migrated foreigners to act as mercenaries, including William Iron-Arm, who rebelled and took a large western chunk of the empire for himself. After a catastrophic defeat at the Battle of Mankizert which involved Byzantines, Turks, Arabs, Normans and Varangians all fighting, the emperor was deposed, and it became apparent to all that the empire was in rapid decline.