Very simplistically, the era of Rome can be split into the time of the Republic with its expansion around the Mediterranean and the Imperial period with its civil wars and northern expansion. The latter period is covered by several army lists in DBA including II/56 Early Imperial (25BC—197AD), II64a Middle Imperial (West) (193-324AD) and II/78a Late Imperial (West) (307-425AD). Roman elements can also form the core of other lists such as the II/83a Patrician Roman (West) (425-493AD) or even II/81a-c Sub-Roman British in the early years.
The Roman army went through a lot of change during this long period and the DBA lists reflect this. As a result my Imperial Roman collection is much larger than needed for any one particular army. In fact, with a few additions of mercenaries from the Northern Barbarian collection there are enough elements for a Roman civil war.
The models are from a mix of sources with Corvus Belli being the main choice.
Knights: Later Imperial armies started to use mounted troops as dedicated shock units (Knights in DBA) rather than fast moving flankers (Cavalry in DBA). The element with the General represents a traditional Middle Imperial heavy cavalry unit (uses a Knights in DBA usually) while the Equites are a 4-model Late Imperial Knight unit.
Cavalry: The more traditional role of Cavalry is resented by Early Imperial Cavalry elements with the traditional stabbing spears. Light Horse elements are not in my Roman collection. Instead, I draft them from the Northern Barbarian collection as they were historically (e.g. Gallic horsemen).
Blades: The core Legion is made up of Blades, including an element with a General for those Early Imperial lists.
Spears: The Imperial period didn’t really use Spearmen but the models are a decent match for Blades of the late period (e.g. II/83a Patrician).
Heavy Auxilia: The Imperial Romans relied heavily on subjugated nations to provide the bulk of their military manpower. The Auxilia formations represent two such units.
Bows: Bows are starting to appear in the Middle Imperial period as a separate combat arm.
Psiloi & Engineers: Unlike the Roman Velites of the time of the Republic, the Imperial Psiloi were mostly mercenaries such as Cypriot Slingers or Gallic spear thrower. The Engineering element is just left-overs form the Roman Camp Follower element could be used as such.
Artillery: The Ballistae and supply cart are the Artillery element of the army and is appropriate for the entire Early to Late period.
Camp: I designed the Roman camp to be usable as either two individual camps (for Roman civil wars) are a combined Built-Up Area (BUA). Both camp bases have a removable Camp Follower element.