Hello. Like all military history geeks, I have a bit of a love affair with the Roman Legions. And it is always good fun to imagine what kind of pseudo-Legion you might come up with... Partly the motivation is my own roleplaying campaign world which, like most home-brew fantasy worlds, has a Rome-analog.
I welcome comments on the names (they try to retain the Roman feel, although I will use some differently from the historical context) and pretty much everything, to be honest. I don't claim to be Gaius Marius and I would be happy to be corrected if I have some blindspots.
1 contubernium (tent group) = 8 men + 2 decurions = 10 men
1 maniple (handful) = 3 tent groups + tesserarius (guard commander) = 31 men
1 centuria = 3 maniples + centurion + optio + doctor + master carpenter + armourer + signifier (standard bearer) + cornicen (horn blower) = 100 men
1 cohort = three centuriae + tribune = approx. 300 men (might be some supernumeraries, like engineers)
1 vexilatio (detachment) = three infantry cohorts + cavalry centuria + prefect + signifier (for the vexilum) + cornicen = approx. 1000 men
1 legio = 10 vexilationes + legate + camp prefect + 2 prefects of the horse + aquilifer (eagle standard bearer) + lots of others = approx. 10 000 men
As you can see, the progression is roughly logarithmic, and I haven't bothered to nail everything down at larger formations.
Vexilatio is basically a 'mini-legion' in its own right, and can be used to garrison a place, or even fight small skirmishes on its own, respond to raiding parties and so forth.
One vexilatio has three infantry cohorts and a cavalry centuria.
I cohort: heavy infantry, equipped with lorica hamata, scutum, gladius, 2 pila, but also trained in the use of a pike
II cohort: heavy infantry, equipped with lorica hamata, scutum (w/ folding spike), gladius, 2 pila, and a sling
III cohort: light infantry, equipped with leather armor, bow, gladius, scutum (with a folding spike to jam it upright into the turf, when need be), pike (probably no more than basic training).
I cavalry centuria: might be subdivided into 1 maniple of heavy cavalry and two maniples of light cavalry. Although I am tempted to make them medium cavalry and keep true heavy/light cavalry as auxilia. If they are medium, then something like shield, lorica hamata, sword, lance and, I am tempted to add, crossbow, might be their equipment.
TACTICS (or rather, how these legions are supposed to use their weapons)
1. Facing infantry
Cohorts I and II fight as Roman legionnaires did. Cohort III would act as archers and might provide a flanking force with shields and swords.
2. Facing (heavy) cavalry
Cohort I would trade the javelins to the pikes from Cohort III and form a phalanx. Since pikes take two hands, they would need to discard those heavy scutum shields. The first rank might retain shield and sword combo, instead, to provide more protection against frontal missile attacks. (Does this sound it would work?) The phalanx might be rather thin, like 5 ranks, but solid. Cohort II would be throwing javelins over the heads of Cohort I and protect the flanks, while Cohort III would be mostly archers but use those javelins as spears and/or to throw if the cavalry manages to get to them.
3. Facing light cavalry / skirmishers
With their big shields and relatively good armor, the legionnaires would probably be able endure a lot of arrows and such from skirmishers. In addition, Cohort II would start using their slings, while the archers would likely get the better of their opponents thanks to the shelter of their shields. Cohort I would act as a infantry moving fort, behind which the other elements can take shelter if need be.
4. Combined army
This is of course the more realistic option. It would be up to the general to estimate what is the best combination of weapons. For example, one centuria of Cohort I might be armed with pikes in a thin (2-3 man?) formation in front of the other two centuriae to discourage a cavalry charge. But if the infantry advances, the pike centuria could fall back behind the other two.
What about their own cavalry?
Unlike the classical Roman legions, these troops would have a rather substantial cavalry arm, too, 10% of the army. This cavalry might be used to contest the enemy's cavalry, or threaten the flanks of the infantry, or chase the skirmishers off the field, depending what they are facing. They wouldn't be expected to outskirmish the horse archers or face off against knights with lances, but they might do well in peppering the knights with crossbow bolts and/or flank knights already engaged by infantry, or to keep enemy light cavalry from having free reign in the rear of the legions.