Battlefield consists of square tiles, with attacker starting at the bottom and defender at the top. You are given opportunity to place your units before battle. Amount of available space depends on number, size and type of units. Starting distance from enemy depends on number of units. More units result in further starting point.
Combat in Planar Conquest is a result of mixing ideas from Master of Magic with d20 RPG rules.
As an example we get number of figures in each unit, each with their own health pool (MoM), and each figure gets number of different attacks with their separate to-hit bonus and damage range. Each attack means each figure rolls separate to-hit and damage roll for each melee or each ranged attack. As a result spells that give you 1d2 or 1d4 damage per hit can give you something like 8-16 or 8-32 damage, if target unit consists of 4 figures with 2 attacks each. Damage resistance works differently: it reduces resulting total damage by resistance value.
To-hit rolls are 1d20 + to-hit bonus rolls. To hit, this roll needs to be equal or higher than target's AC. As a result each +1 bonus to to-hit corresponds to +5% (1 in 20) chance to hit. Each +1 to AC reduces chance to be hit by -5%.
Saving throws follow the same logic. Saving throw is a d20 roll + saving throw bonus, and to succeed this must result in number at least equal saving throw DC.