Cover

When the origin of an attack or the center of an area effect is in a square meeting the criteria that at least one line traced between any two points of both squares is impeded by a solid obstruction (such as a wall, not a creature), then the defender of the attack is considered to have partial cover.

• Attacker suffers 50% penalty to Hit Chance
• Defender gains 50% resistance against area attacks

When it is not possible to trace a single line from the attacker's square to the defender's square that is not impeded by a solid obstruction, but there exists some conceivable way to attack, such as through narrow arrow slits in a solid wall, the defender has near-perfect cover.

• Attacker suffers 100% penalty to Hit Chance
• Defender gains 100% resistance against area attacks

When it is not possible to trace a single line from the attacker's square to the defender's square that is not impeded by a solid obstruction, the defender has total cover.

This makes it impossible to attack the defender.