How do attack values work? Attack + lethality, armor/shield/etc

[The Celtic Viking]

Armour counts in all directions, but against armour piercing weapons only half of it is counted.
Defensive skill counts to the front and right, but not at all against missiles from any direction.
The shield value counts from the front and left, and is counted double vs missiles from the front.

As for attack vs lethality, I think Paltmull is correct. For the same reason, against units with low defense value, a high lethality is worth more than a high attack, while more defensive units are better met with a higher attack (though AP is even better, of course). Then attack speed can come into play, but I think only the cavalry lances have that to such a degree as to be worth noting.

But that's only speaking of green units. If we take experience into account, then a high lethality becomes ever more valuable, as the attack value increases but lethality does not. You're stuck with whatever lethality you start with, while the attack value is steadily devalued. With the examples you give, the potentials are 18/.225 and 26/.11; the former is clearly the better, because it easily has enough attack to land hits on a regular basis, while it has more than twice the lethality than the latter.

There is a guide to the EDU in the Scriptorium. Here's the link.

Edit:

Hmmm...

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 don't think Paltmull was talking about defence skill at all, but rather the full defence value of armour, skill and shield added up together. The reasoning being that, since only the armour counts from the back, a unit attacking from the back need less in attack to land hits, and would thus profit more from being able to kill more often when landing hits: ie, higher lethality.

On the other hand, when attacking from the front you need to pass armour, shield and skill, so from there a higher attack becomes more important.

In other words, units with high lethality weapons are absolutely fabulous against Gaesatae when sticking their pointy things in from behind.