Units will have Offense and Defense. Offense is attack damage; Defense is health. If a unit is not utterly destroyed in a battle, it will be at full defense for the next one. That is, I don't keep track of health.
There will be certain tags applied to units. Note that these are unrealistic and abstracted, the main goal being ease of use.
Attack type: Attackers do double damage to those weak to them.
HACK: Hacking units are generally those that use swords, axes, etc. They are weak to piercing damage, generally having poor armor against it. However, they excel against crushing units.
CRUSH: Crushing units are the heavy units. They have thick armor, which renders piercing damage almost useless. However, hacking units are speedy and accurate enough to find weak spots, yet have strong enough weapons to deal serious damage.
PIERCE: Piercing units are bowmen, spearmen, etc. Their advantage lies in their strong capability to strike deeply - an arrow to the gut can fell a swordsman easily. However, their blows are quite weak and the thick armor of crushing units will repel them.
When two armies meet in battle, damage will be calculated. First "critical" damage will be calculated for each type. For each type, the amount of possible critical attacks is the lower of how many units of that type you have, and how many appropriate units the opponent has. The most powerful attacks you have are added for the critical damage for that type, then doubled. After all the types have been calculated, the remaining attacks are added to calculate standard damage. Each type of damage is multiplied by (4to12/8), meaning 50% to 150% randomly. For each type of damage, the highest-health enemies are killed until you run out of damage. Then the next round starts. This sounds more complicated than it is - here's an example.
I have 3 swordsmen and 4 spearmen. The enemy has 2 swordsmen, 1 maceman, and 2 spearmen. Their stats are all (1, 4).
One of my swordsmen matches up with and counters the enemy maceman; two of my spearmen counter the enemy swordsmen. The 4 remaining units deal standard damage. I roll 8s on everything. Thus I deal 2 hack, 4 pierce, and 5 standard damage.
The enemy maceman counters one of my spearmen. The enemy spearmen counter my swordsmen. The remaining 2 units deal standard damage. The enemy rolls 12s on everything. 3 crush, 6 pierce, 3 standard.
My 2 hack cannot kill the strongest unit, so it is saved till next turn. My 4 pierce kills an enemy swordsman. 4 standard damage kills a random unit - say, a maceman - and one is saved for next turn.
The enemy 4 pierce kills one of my swordsman; the rest is saved.
And so on.
Some more tags are:
RANGED: Cannot be attacked. If all other units die, the ranged units retreat to their original country.
FAST: Can travel 2 countries in one turn.
NOMOVE: Cannot travel.
DEFENDER: When defending, is not susceptible to countering.
Units have costs in turn values. A country may be set to produce a unit, and it will spend the requisite number of turns producing it. If the unit is cancelled, the production is wasted. Unit production cannot be transferred between countries.
If you have five or more countries, it is possible to consolidate two countries into one. Both countries must not border any country that you do not own. This takes two turns and creates a new country that combines the production of the two. You may do this multiple times to create countries with very high production, provided you fulfill the requirements.
Units can generally move one country a turn.
Spearman: 1, 4: 1 turn: PIERCE
Swordsman: 1, 4: 1 turn: HACK
Maceman: 2, 8: 2 turns: CRUSH
Town Watch: 1, 5: 1 turn: NOMOVE, DEFENDER
Catapult: 4, 8: 3 turns: CRUSH, NOMOVE, RANGED, DEFENDER
When probed, neutral countries roll a d4 to determine how many defenders there are.
They will then roll a d8 to determine the degree of countering. 8 means the defenders counter your units exactly; 1 means they are countered exactly. 5 or above means the defenders are on High Alert, which means your units' counters do not work. 4 or below means the defenders are Caught Unawares, which means that their units' counters do not work.