afaik armor value is multidirectional for melee and ranged, and is the "only" thing affected by AP-weapons as their name suggests. Shield is left and frontside for melee and ranged. ddefence is to be seen more of a parrade than a dodging action and thus is only for melee and the front and rightside(where the weapon is located)


So units with high lethality (ie. those crazy 2-handed swordsmen) = bad against units with high defense skill (ie. Gaesatate)
especially since high lethality usually goes with AP bonuses, and Gaesatate wear no armor anyways...

High lethality + AP bonus = good against units with higher armor, but lower defense (post-Marian cohorts)
I would agree with this especially as high armor value often goes with relatively low defence value, still high lethality is always of use :D

Still a information on the exact math with the attack value vs total defence would be more than welcome.