Unit depth and defense

[hamysho]

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

Is there an optimal 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?

And the difference between armor and defense skill. Does armor add to your melee defense for a cumulative damage reduction or is armor just used against missile attacks.

Thank you!