Basic Values and Stats for Anuirean troops:

Archers have gets +1 on Plains (Long bows have better affect when it not so much in the way)
Archers get -1 in Woods (way to much in the way :)
Archers get +1 in defence in Mountains (fire downwards...and they're not so easy to catch)
Archers can't be used Underground.
Infantry gets +1 in defence in Hills and Mountains
Elite Infantry get -1 in attacks in Woods and Mountain.
Elite Infatry gets +2 in defence in Mountains
Pikemen get +1 in defence in Plain, Hills and Mountain for each oppousing cavalry unit (max your own pikemen number).
Irregulars gets +1 in Woods.
Cavalry gets +1 in Plains.
Cavalry gets -1 in Woods and Marches.
Cavalry gets -1 when attacking in Hills.
Cavalry can't be used in Mountains or Underground
Knights gets +2 in Plains.
Knights gets -3 in Woods.
Knights gets -1 when attacking in Hills.
Knights can't be used in Mountains, Marches, or Underground.
Scouts get +1 in Mountains.
Artillery can't be used in attacking Mountains, or Marches.
Levies gets -0.5 in Plains.
Levies can't be used Underground.

Add up the the modified values of each unit to find your army strenght.
If characters are leading the battle, they add bonuses ass follows:
Warrior Class: +1 pr character level
Rogue Class: +1 pr 4th character level
Magic Users: +1 pr 6th character level or if they only cast spells, see the below.
Successful strategy NWP: +5
Battlewise: +1 for each unit in your army
Can Turn Undead: +1 for each enemy undead unit.

Notes:
If a priest faces undead in battle, he must choose whether he will spend time turning them or
casting other battle spells. Consequently, the priest only gets one bonus - either for his spells
or for the undead turning ability.

For multiclasses characters, apply the best class bonus in full, half of the next best one,
and nothing for the third class (if any).

Character bonuses of spellcasters

A spellcaster has a base bonus equal to the number of spells he can memorize.
Thus, a 5th-level wizard (4/2/1) would be worth 4+2+1=7 points. Bonus spells received by priests with high
Wisdom are counted into this value. Note that a spellcaster must know at least one spell usable in battle at
each level in order to receive the points for that level. Spellcasters receive further modifications to their value
as described below.

Versatility
Spellcaster knows spells from 2 or fewer categories -5
Spellcaster knows spells from 3-4 categories normal value
Spellcaster knows spells from 5-6 categories +5
Spellcaster knows spells from 7-8 categories +7

The categories include Transmutations, Fogs, Massmorphs, Walls, Blesses, Hallucinatory Terrain,
and Attack Spells in land battles; Crew-Affecting Spells, Illusions, Fogs, Movement Spells, Barriers,
Bless, Turn Wood, and Attack Spells in sea battles.

Spell Power
For each researched improvement to battle spells +1
For each known "D"-result spell +1
For each known spell in War sphere +1
Note that this bonus considers the spells that the spellcaster has in his spellbook
(or that are granted to him by his God), not the spells he has memorized.

Enemy Magic Resistance
If at least 50% of the enemy unit is immune to F and R results from spells, the spellcaster's total value is reduced by 25% for that battle.

Situational Modifiers

These are applied to the total army strength before each battle, based on the situation.
Note that these are applied simultaneously, rather than in order given; thus, all percentages are counted off
the base army value (including character bonuses).

Troop Ratio (applied to the larger force):

Morale (use all that apply):
+5% if in home realm
+5% if have beaten this foe before
+10% if attacking an enemy which has used its full movement point allotment to get to the battle
-5% if any friendly armies were defeated in the same war move

Environment (use all that apply; note that this is not a terrain bonus):
+10 if extremely favorable environment (e.g. orogs at night)
-10 if extremely unfavorable environment (e.g. Khinasi in a snowstorm)
+10 if the entire force has infravision/night vision in a night/darkness battle
(if the enemy force also has it, this bonus is negated)

For attackers, when assaulting a castle:
+2 per artillerist unit
+2 if Engineering NWP is available
+5 if Siegecraft NWP is available

For defenders (defender determined as per BR war move rules).
These modifiers are ignored if all attackers can fly, and halved if at least 10% of the attackers can fly.
+20 if defending a narrow defile, pass, or bridge
+15 if attacker just crossed a river to get to the province
+10 if defending in other rough terrain, or behind a wall
+3 per castle level for each unit defending in a castle or fortification

Immunities (use only one per force):
+50% if the force is completely immune to enemy attacks
+20% if at least 10% of the force immune to enemy attacks
+20% if the force is immune to a significant number of enemy attacks

Diversity:
+10% if the army consists of three or more general unit types (mounted, missile, melee, flying missile, flying melee, etc.)
-10% if the army consists of only a single general unit type

Note that some units can be considered as belonging to more than one general type
(e.g. irregulars are both melee and missile). In that case, the army commander can assign them to different tasks
and avoid the penalty. However, he cannot gain the bonus in this case either (because mounted and melee
are mutually exclusive, and flying and walking are also mutually exclusive; thus a single unit type could have
at most two functions).

Tactics (optional)
Each side can submit a battle plan. The plans are then compared and modifiers are applied.

Here are the "general" battle plans:
Full charge: all units will rush forward to attack.
Normal attack: units will move forward, using both melee and missile combat.
Encircle: units will try to move along the sides, encircling the enemy.
Trap: units will present a weak center and try to lure the enemy into moving forward; then they crush him from the sides.
Hold: units will maintain a strong position and avoid moving forward.
Withdraw: units will retreat at the first opportunity, trying to avoid the battle altogether.