Pre-Reform - Generalist with a mix of light and heavy infantry, archers, skirmishers and light cavalry.
They increase the odds of casualties healing after a battle and improve the fertility of that General.
Combat Medic: Physicians are a common ancillary for Roman Generals to pick up.
Color-Coded Armies: The Brutii are green, the Julii are red, and the Scipii are blue.
The Clan: Each faction is based around a single influential family of the Roman Republic.
Formations are key to most Roman units, and they become much less effective and more vulnerable when out of those formations. Public order in cities with these structures is boosted, and can be further improved by increasing the frequency of these events (at a high cost to the city's revenue).
Bread and Circuses: The Roman factions are unique in that they can construct gladiatorial arenas and chariot racing tracks.
Following the Marian Reforms, the entire unit roster of the Roman factions gets a major upgrade, but their overall strategy remains the same. They possess skirmishers and cavalry which are useful for screening enemy forces and protecting the flanks, but are well below the quality of equivalent units in factions who specialize in them.
Boring, but Practical: Much as it was in reality, the main battle tactic of the Roman factions is to send a line of quality heavy infantry at the opponent and overpower them.
(In the case of Town Watch, this is essentially the only thing they are good for.) Both make for surprisingly good defensive troops. Their purpose is to reinforce the legionaries at the front of the line and protect their vulnerable flanks against cavalry attacks. The Reforms also add Auxilia, who use spears and who get even better armor and training. They remain on the roster even after the Marian Reforms. Given short spears and basic shields, they are essentially slightly better equipped and trained peasants.
Blade on a Stick: The most basic non-peasant unit in the Roman faction roster are Town Watch units.
House Scipii = Gens Cornelia - The family of the Roman General and Consul Scipio Africanus, famous for defeating longtime Roman enemy Carthage.
Julius Caesar is their most famous member, and it was this family which established the Julio-Claudian dynasty of the early empire.
House Julii = Gens Julia - Was one of the most prominent families in the time of the early republic.
House Brutii = Gens Junia - Came into several high governmental positions in the late republic and early empire.
Badass Family: Each is based on a Real Life prominent family from the time period the game takes place.
The high quality of armor worn by Roman heavy infantry is what turns them into some of the best units in the game. Even out of that formation, Roman infantry tends to be difficult to damage with missile fire due to their large shields and quality armor.
Annoying Arrows: A specialty of the legionnaire units is the "Testudo" formation, which basically allows them to move (very slowly) completely unimpeded by missile fire.
Ancient Rome: Each represents a noble family from the Real Life late Roman Republic era.
This is in spite of you each having mutual interests (at least until the civil war).
Aloof Ally: Despite starting out allied with eachother (and the Senate), it is just as difficult to get them to seriously assist you in any meaningful way as it with a faction you choose ally with yourself.
100% Adoration Rating: The closer a Roman faction comes to achieving it with the people, the more the Senate will begin to dislike the faction out of a fear that they may try to take over Rome.
They initially report to the Senate, but their victory conditions include the eventual overthrow of the Senate and conquest of Rome itself. Each starts with two cities under their control: The Brutii at the southern end of Italy, the Julii in the northern end of Italy, and the Scipii with one just south of Rome on the Italian west coast and one in Sicily. The three initially playable factions, representing the Brutus, Julius, and Scipio families of ancient Rome, respectively.