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I have compiled a list of the currently available military units, and ones that may be researched, detailing their advantages and disadvantages. Feel free to add units I have missed out, as long as they existed at the time.
ARCHERS
There are two types of archer and one yet to be researched.
To be effective on the battlefield, bowmen must be protected by other units or stationed at locations the enemy cannot reach. Bowmen, with the exception of crossbowmen, must train for years to become proficient with the bow.
Regular Bowmen
Equipment: none or light armour.
Primary Weapon: Bow & Arrows.
Secondary Weapon: Dagger
Pros: long range, rapid fire.
Cons: weak in close quarters combat.
Cost: medium/low
Long Bowmen
Equipment: none or light armour.
Primary Weapon: Long Bow & Arrows.
Secondary Weapon: Dagger
Pros: long range, rapid fire, can pierce light armour.
Cons: weak in close quarters combat.
Cost: medium
Crossbowmen (not currently available)
Equipment: light armour.
Primary Weapon: Crossbow & Bolts.
Secondary Weapon: Dagger.
Pros: medium range, can pierce heavy armour, capable of hitting moving targets.
Cons: slow reloading, weak in close quarters combat.
Cost: medium/high
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KNIGHTS
Knights are heavily armoured and highly-trained soldiers. In Western Europe only royalty, noblemen and their private soldiers could train to be knights. No peasants or labourers could become knights.
Single-Handed Swordman
Equipment: Medium Armour.
Primary Weapon: Sword & Shield.
Secondary Weapon: Dagger.
Pros: very good in close combat, can use the shield for defence against arrows and light weaponry.
Cons: weak against heavy weaponry, fairly slow moving.
Cost: High.
Double-Handed Swordman
Equipment: Heavy Armour.
Primary Weapon: Long Sword.
Secondary Weapon: Dagger.
Pros: Heavy armour. Can cut a man in half even through armour.
Cons: Very slow moving.
Cost: High.
Cavalry
Equipment: Horse, Medium or Heavy Armour.
Primary Weapon: Lance.
Secondary Weapon: Sword.
Pros: Well armoured, fast moving. Strong against dispersed troops.
Cons: Vulnerable to pikemen. Weak against compact, organised troops.
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SIEGE WEAPONS
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PEASANTS
Using assorted and improvised weapons; mostly agricultural tools. Untrained soldiers but capable of swarming on mass with great effect.