Front Line Units: Units with higher 'Weakness' always receive damage first in all 'Waves' of attack. Lancers come in to play here, as well as other units like Eagle & Jaguar Warriors, but for the most part - throughout the game - Lancers are going to be your driving force and your fodder. The exact number at which 'Weakness' comes in to play when the combat calculation beings is currently unknown, but to be safe it's always good to have high numbers of Lancers. You have to make sure that you always have them when attacking, if you send only Bowmen at an enemy, they will be slain before they can output any casualties to the enemy fortifications. Though most of this may come as common sense once it's read/realized, it's still good to keep an equal amount of units within your army, always keep it diverse.
•Weakness - The percentile value of the Attack & Defend priority of a unit. Exact Threshold of when this takes place is unknown, however, I assume that the equation works similar to this; Units with 51% or More Weakness will both Attack & Defend in Wave 1 of a Battle. Units with 50% or Less Weakness will both Attack & Defend in Wave 2 and on of a Battle.
I'm not at all convinced that this is how it works. The leading player in the game is cheerfully and successfully attacking without lancers or bowmen.