Unit depth and defense

[al Roumi]

Sorry if this a newbie question but this is my first Total war game since shogun 1.

Is there an optional depth for melee and ranged units? And what's the reason for depth, will a unit that has a thin line rout faster than a unit that's deeper?

I assume you mean optimal depth? I don't think there really is as it's as much a function of the unit's ability, size, the enemy it will face and your tactics.

I've not noticed archers having any difficulty with direct/indirect fire due to their formation (as in MTW2) but I'd imagine you'd want as broad a frontage to your teppo units as possible for more shots per volley (i've not used any yet but I doubt they have fire by rank or other drills).

When it comes to melee, I'd say it depends on the unit's role. A unit designed to charge and break through opponents will want a resonably deep formation. Similarly, a unit designated to defend against a charge will want some depth to keep its place in the line. Then again, your line does need to be broad enough to fix the main enemy charge and prevent enemy flanking whilst also allowing for your own flanking movements.

While what I mention about charging units will also hold for cavalry, it's worth remembering that a unit in a tighter formation (i.e. like a square/column) should maneuvre faster than a unit deployed in a line. This was certainly the case in Empire and other TW games but I've not noticed it to the same degree in TWS2 due probably to the (comparatively) high unit movement speeds.

[Zarky]

(i've not used any yet but I doubt they have fire by rank or other drills).

There is fire by rank available when you're standing still, so best depth for teppo units is probably 2-4 lines depending on the battlefield.
I don't think there is one optimal depth for other units, cavalry will probably smash through any unit that is stretched too thin even without wedge formation.
I use rather thin lines because I've never ran into a situation where I'd be critically out of space, where deep lines would be more beneficial or where I was at risk of being charged by cavalry.

I suppose it depends mostly on your style of play.