Armour in Woods or Towns

[Steel God]

Armor and Vehicles generally are penalized when attacking INTO non-clear terrain (50%) if not supported by sufficient amount of accompanying infantry.  They must have 10 infantry for each vehicle, and they total infantry must outnumber the defending infantry to avoid the penalty.  If they're not all from the same organization there's a separate penalty as well.

In regards to artillery fire, since your artillery shouldn't be exposed to too much fire (if they are you need to move them) terrain shouldn't matter too often.  If a unit is on the attack, setting up and moving are probably the higher priority, so clear terrain rules.  Defensively, especially if you intend to make a stand of any length, find terrain of defensive value to aid against inevitable counter-battery.