Well, it's a system I already made for another game, so it's actually a case of just using certain mechanics for this one. Here are the choices.
As-Is
One Unit is one unit. In fights, whoever has the most units wins, both sides typically lose one or more units. Very abstracted, very low fidelity.
Basic Complexity
All player units are the Tail Swordsman as below, working with these mechanics:
Tail Swordsman
A sword-wielding infantryman typical of the states of Lucindra's Tail. Wears a bronze helmet, light chainmail with shoulder plates and greaves and a large shield, fighting with a sword.
The above unit has 8 'figures' in comparison to a barbarian unit of 12, but the same amount of effective health. Each figure gets its own attacks against an enemy unit's armour, based off a roll of 2d6, and being non-barbarian the swordsmen get an attack and defence bonus for discipline. Survivors get to level up, increasing attack and effective armour.
In this setup, armies fight as two sets of AI-controlled units operating off simple decision trees (very simple, as everyone's a melee infantry unit), there is increased likelihood of lethality on both sides, but it is possible to win without losing units.
Units cost 1500 as before (with Powerful discount), and 100 upkeep.
Intermediate Complexity
There are a few player unit types that can be recruited. By default, all existing player units become Tail Swordsmen as above. In addition, there are the:
Tail Spearmen
Medium infantry, typical of the states of Lucindra's Tail. Wears a bronze helmet, chainmail with shoulder plates and greaves and grips a 6' spear with both hands. It won't stop a cavalry charge outright, but any survivors will give any equites a very bad time. [Slightly less armour, effective vs. cavalry.]
Tail Skirmishers
Light infantry typical of the states of Lucindra's Tail. Wears leather armour, carries a shortsword and several javelins to throw. Very mobile compared to other infantry, but easily hit.
Tail Archers
Light infantry typical of the states of Lucindra's Tail. Wears almost no armour, but sports a hardwood shortbow and arrows, with only a dagger for close combat. Good range, very mobile, death at range, doomed in melee.
Equites
Light cavalry drawn from the equestrian classes. Wears light chain and a bronze helmet, carries a small shield and a spear - can execute high speed charges to crush infantry and has a chance to disengage from fights to make another charge. 6 figures per unit, ruinously expensive to recruit and maintain. Exceptionally fast.
As one would expect, this means that unit composition has a bigger effect on army choice and introduces another layer of strategy. As mentioned, the engine already accounts for this.
Advanced Complexity
I let people go nuts with designing their own units. Not recommended for this game.