Skeletons are simply animated bones, brittle and rather weak in battle. Their only saving grace is that once summoned, they require no upkeep*.
\*- they actually do require 1 mana upkeep.
Summon - Unit was summoned and belongs to another world.
Bludgeoning Vulnerability - Unit suffers 25% additional damage from all Bludgeoning attacks.
Illusion Immunity - Unit is immune to illusions. It can see invisible units and illusory attacks do not ignore its armor
Piercing resistance - Unit receives 5 less damage from all Piercing attacks.
Undead - Unit is immune to Mind and Death effects, poison and sneak attacks. In addition they suffer damage from Positive energy and are healed by Negative Energy.