Armor is able to be activated using souls when your building reaches a certain level. Armor is used to prevent enemies like Pan Goli from one hit destroying your buildings. Unless you level the building armor up, it really won`t make a difference in HP but it will prevent a enemy for destroying it in one shot. For example if a sniper tower is level 11 and gets hit by a high level hero skill it might be destroyed in one shot but if you activate the armor using souls for the same sniper tower level 11 the enemy will use his skill and break the armor barrier then have to attack the bulding as normal to destroy it preventing one shot destruction and giving your other towers and heroes that extra second or so to attack the enemy attackers.