Determining how to model forces across the board in this kind of game is not a trivial matter. Standardized units did not exist; not even within the same state, so just settling on a unit size becomes a problem. This is closely followed by the issues of determining unit factors, how strong they should be, and so on.
In Imperium, the “base unit” will be the so-called “regiment”. It represents a variety of different types of units, but essentially a regiment numbers somewhere from 500 to 5000 men, with the typical infantry unit being 4000 men, and the typical cavalry unit numbering 2000 men. In other words, a regiment is equivalent to a full Roman legion. Units can take losses (manpower factors), so it will be possible to have half-strength units.
Regiments have a quality (from raw recruits up to legendary), a fighting doctrine, and a special trait. Quality is the “experience” of the units, ranging from raw recruits to legendary. Fighting doctrine is the system used to distinguish between special units (e.g., beasts – elephants and camels, chariots), cavalry (shock and missile), and the few distinct types of infantry (shock, missile, and phalanx).The special trait provides the unit with some kind of bonus in specific situations (more on that later). In addition, each unit is distinguished by three factors used in battle resolution: their combat value, their skirmish factor, and the scouting factor.
The combat value determines how well the unit fights in set-piece combat. The heavy units (roman legions, heavy cavalry) are the usual suspects when it comes to scoring highly in this category. Quality is also important however; in battle, the combat value is multiplied by the quality of the unit. This makes it possible to reenact events such as the battle of Pharsalus; 10 half-strength veteran legions will be more than a match for the same number of full-strength raw legions.
The skirmish factor determines the effectiveness of the unit in a skirmish. The skirmish factor is useful in different contexts; one of the results of a confrontation between two armies can be a minor skirmish instead of a pitched battle. Also, in a pitched battle, there is usually a skirmish phase in which the skirmishers can attempt to inflict losses on the battle line troops (and more frequently, each other). Finally, the skirmish factor is used during ambushes.
The scouting factor is the final piece in the force model. It is used to determine which of two sides in a conflict have the better intelligence. If one side “outscouts” the other by a significant margin, it will be able to attempt an ambush of their opponent. How much of a margin is required depends on the terrain; thus Romans campaigning in Spain and facing Scutarii and Caetratii units with high scouting factors will have to be extraordinarily careful in their operations. Light infantry and cavalry units will typically have a high scouting factor; while units made up of heavy infantry will struggle.
As an example, consider the early Roman consular army. It would consist of one cavalry unit (2000 men); too little to effectively defend the Roman army’s flanks properly (as occurred historically, the player will soon find himself recruiting cavalry auxiliaries). In addition to these, however, it would number four manipular legions (16,000 men). Manipular legions are among the best battle line units in the game; with a very high combat value. In addition, the manipular legions possess the “Skirmish Detachment” special trait (representing the velites). The effect of this trait is to have the legion automatically count as 1 skirmish factor during the skirmish phase. As a typical skirmish unit will have a skirmish factor of 4-5, this means that four manipular legions essentially match 4 heavy units + 1 skirmish unit. This makes a Roman army very effective; its only achilles heel is the lack of scouting ability as the lone cavalry unit will be insufficient.If the Romans carry out their military reforms, they Legions will dispense with the “Skirmish detachment”, but instead gain additional combat value. This makes the Roman legions even more formidable; but only if they can be supported with good quality cavalry and light infantry. A typical army of the other powers would contain two or three cavalry units, four heavy infantry units, and one or two skirmish infantry units.
I hope the model will permit reasonable simulation of most of the different troop types that would be included in the game, and most units have their place in the framework. Heavy infantry provide the fighting punch; cavalry are required for fighting on the flanks and scouting, while light infantry are required for skirmishing duties (cavalry can skirmish quite well, but are very expensive to recruit and maintain in comparison with the latter).