So is each unit a hit point, or what?
Each guy in a unit, called a figure, is basically a hit point bar. If a unit has 3 hp, then every 3 hp of damage it takes kills a figure(healing that will revive dead figures). Total Attack stat(and, technically, HP) is multiplied by the number of figures(example, Spearmen with 1 Attack but 8 figures effectively has 8 Attack). Losing figures can effectively weaken a unit. It's worth noting that although a hydra(Nature summon) appears to have one figure, each head actually its figures(which, combined with their abilities) makes them lethal.
Every point of attack(called "swords" for short) is an attack roll. Each roll has a 30%(+10% for every To-Hit bonus) to hit. Then, every sword that hits goes against the defender's defense rating(called "shields"). Each shield makes a roll to negate a hit(30% each roll, 20% if attack has Eldritch Weapon). Each successful attack roll must make a roll against each and every shield. Every attack roll that makes it past the defense inflicts 1 HP of damage.
It's a bit more complicated. 8 spearmen with 1 attack each don't actually deal 8 attack. They do 8 attacks worth 1 point.
now let's assume we have a creature with 4 defense being attacked by 8 spearmen OR a creature with 8 attack.
The spearmen in theory can do the same damage BUT each point of damage has to go through 4 shields (each having a 30% chance of blocking the damage) while a creature with 8 attack will ALWAYS do 4 damage and the shields have a CHANCE at stopping the remaining 4 IF they all work (30% chance per shield).
This makes singular creatures with more attack much more effective than a bunch of weaker ones even if they seem to have the same # of swords/shields as a whole.
Since when does 8 attack guaratee 4 points of damage? Was that added in a later patch that I probably don't have?
Well there's the chance of the attack missing completely before defense is taken into account.
My point still stands however. 1 shield can block only 1 sword and if once creature connects more swords than enemy sields the damage becomes unblockable (because there are no shields to block them).
Unlike several weaker units with the same # of swords total.