I remember reading about medieval dueling and often times, duels and battles would end simply because one guy fell to the ground and couldn't get up because of the armor. THe opponent would simply disarm the guy or wait until he stopped struggling then take out his dagger and stick it through the slit in the helm or through a soft part of the armor (such as the armpits to the heart). Rarely would non-large swords actually be able to cut through armor w/o hitting a soft part of it. More often, axes and other armor piercing weapons were the ones able to do so.