Overview:

Factions:

All Will Serve: -50% upkeep cost for all auxiliary units
Cultural Assimilation: -50% public order penalties due to the presence of foreign cultures

Agrarian Wisdom: +30% wealth from agricultural buildings
Founding Fathers: Public order bonus (maximum of +6) from presence of Latin culture

Cultural Traits increase unit recruitment capacity, allowing for rapid recruitment of whole whole armies and navies, and allow armies to move further on the campaign map each turn.
Starts with control of much of central and southern Italy, with easy access to the central Mediterranean, allowing for multiple avenues of expansion. Also controls both a Wonder (Mount Vesuvius, which gives faction wide bonuses to agricultural income and unit morale,) and a Special Region (Roma, which increases income and army recruitment capacity in the Italia province.)
The Roman roster comes in two parts. A core roster of Roman units recruitable in any province with the right military buildings and with a focus on melee infantry. And a much larger Auxiliary roster, which draws from the local populace of a province, with the exact units available to be recruited depending on the province. This creates a roster which is both very versatile, but also largely dependent on what provinces the faction controls.
Instead of separate factions, faction choice decides which party the player will use to lead the Rome faction, with each having different Faction Traits.

The dominant power on the Italian peninsula, destined to become so much more.
Though Rome may have started as collection of settlements on the banks of the Tiber river, in this era it would expand to become an empire stretching from the Atlantic Ocean to the Persian Gulf. It's influence on many aspects of the world we live in remains to this day, for example the use of Latin in certain professions, such as the law.
The Roman military's strengths lay not only in its organisation, discipline and strict tactical doctrine, but also its ability to adapt to change. It adopted the weapons of its enemies, such as the famous gladius (full name “gladius hispaniensis” or “Hispanic sword”,) a short stabbing sword designed to thrust into the enemy to injure or kill. It also adopted those enemies themselves, drawing auxiliaries from other conquered peoples.