I think that keeping reserve melee units behind the front lines is going to be very important in any battle between two large armies. And in most cases, I think you're going to want to keep your best melee units in reserve whenever possible.
On his turn, the enemy can destroy nearly any single unit in your front line if he wants to, through bombardment and focused attacks from multiple units, so it will be key to have units in reserve to plug these holes. And, since you really can't prevent any front-line unit from being killed, you're probably going to want to have the reserves be your most elite units, so that they can provide decisive support and still have a chance to survive the battle.
Similarly, when you're the one doing the attacking and kill a unit in the enemy line, the unit you won the battle with has moved foward and now opened a hole in your own line (and is probably dead meat on the enemy's next turn), so you're going to need reserves to plug the hole at the very least (or continue the attack on other units if conditions permit).
So I think what you will see especially early on when Iron is limited, are formations with spear units in the first rank, and heavy infantry reserves interspersed with ranged units in the second and third ranks, and the odd mounted units on the flanks. The melee reserves can be several ranks deep, allowing for them to use the new two-space movement and unit swapping to jockey in and out of position with ranged units in the critical second rank.
As resources become more plentiful, more and more of your front line can be replaced with heavy infantry, but I think the wise commander will keep his most elite units in reserve.
Nicolas10 said:
I think it's interesting that people are talking about such large groups of forces...
I'm seeing some pretty big armies in these preview screenshots.