Terrain

All provinces in the game have a set of terrain tags. Terrain influences provinces in many different ways:

It influences the size of the population in the province as well as available resources and recruitment points.
It determines the frequency and types of magic sites that can spawn.
It determines what independents spawn, as well the type of province defense and independent recruits that players may buy a
It affects the movement cost of units moving through the province.
It can influence units in combat, adding penalties to those not adapted to the environment.

Terrain & Combat

Fighting in certain terrain applies penalties to units in combat.

Swamp

All units suffer -1 attack, -1 defense, +2 encumbrance and -20% combat speed when fighting in Swamps.
Swamp Survival completely negates this penalty.
The spell Quagmire turns any province into a Swamp for the duration of the spell.

Cave

Deep Sea

All units suffer -3 to attack, defense and precision when fighting in Caves or Deep Sea.
100% Darkvision, Spirit Sight or Blind Fighter completely negates this penalty. 50% Darkvision reduces the penalty to -2.
The spell Solar Eclipse creates a similar effect.

Sea

Deep Sea

All units suffer -3 to attack and defense when fighting in Sea or Deep Sea.
Amphibious or Aquatic completely negates this penalty. Poor amphibian reduces the penalty to -2.
Blunt and Slashing weapons suffer a penalty to attack equal to their weapon length. Flails suffer an extra -1 penalty to attack.