Mountains and Valleys uses a simplified classless system. Armed combat is based on a list of weapon skills and a number of stats. Magic is also a very basic setup. The idea is to allow a wide range of possibilities that draw from a few core equations.
The character caries a spell book in which she records all of the spells she has learned. From this, she can memorize up to eight spells at a time which can be cast. Only memorized spells can be cast, and some spells may require up to three slots.
Spells also have a minimum level requirement as well as a minimum 'magic' skill requirement. Other than that, there is no check; if both conditions are met the spell will be cast. All spells are cast instantly; there are no enchantments, just damage or healing spells. There is no DV or AV for spells, if you can cast it the effect happens every time.
All magic falls in to one of four types: Earth, Fire, Wind, and Water. The player or monsters she fights can have points of resistance in each of these. When magical damage is dealt (damage from a spell) the resistance is subtracted before damage applied. For the player, this also effects environmental damage (with a high enough fire resistance you can walk over lava, etc.).
MV will use a simplified AV/DV system similar to Mage Wars RPG. Effectively your various stats come together to form an Attack Value and a Defense Value.
Attack Value is Equipped Weapon Skill + Strength + Equipped Weapon Modifier Stat + Variable Weapon Damage. Weapons have a set range of damage, whenever an attack is made this range is rolled as part of the attack. Each weapon also has a given modifier which is the stat used when calculating AV. Very simply, if the player's AV is higher than the target's DV, the attack lands and damage is dealt.
Now, this can lead to the player encountering enemies that they simply cannot attack; if their highest achievable AV is lower than the monster's DV they are out of luck. This usually happens because the player has entered and area where the enemies are much too high-level. Magic can fill the gap, since there is no AV/DV check involved. Also the player can go back and level a bit more before attacking the next area.
Unlike AV, DV is a static. Its based on Stamina + Level + Armor Class. Equipping better armor or buffing the Stamina will help with DV. DV only determins whether or not you get hit, it is not part of damage calculation.
For a basic attack, damage calculation is done by weapon damage. This is rolled again(separately from AV Calculation) and any modifiers on the weapon added. The target's equipped armor AC is then subtracted, and any remaining damage is dealt.
So armor with higher AC subtracts damage, but equipped armor is not the same as over-all armor class.
Nadine has the option of learning a number of "special attacks". These are condition-based in that a given move cannot be preformed unless certain conditions regarding equipped weapon type and necessary augments are met. These moves are simply hot-keyed and can be activated, though they come with a cooldown. The player can still use other special attacks or normal attack while cooling down.
Special attacks are another form of DV-breaking as they can make it possible to hit an otherwise immune enemy.