Valleys are strips of open land with some sort of obsticle to either side of it. These are the more difficult areas for proper combat and players tend to avoid them if possible. The difficulty lies in the forced density of troops, while highly limiting the effect of infantry. Protecting against attacks coming from valleys is very easy, as you always know where your enemy will come from. When fighting in a valley, you will always have at least some sort of tank support, while also taking something with you scary things that can come from the sides.
There really is no right way to wage war in valleys, as they limit you in almost every way possible. Depending on what comes from the sides, you might not want to move through these areas until you aquire some really heavy weapons. On road to rome, a valley is the center road connecing the top portion of the map to middle - when passing through this area, you are probably going to get ambused by infantry. On village crossing, you can get hit by tanks on the hills if you choose to pass through the center, often leaving your weaker armor exposed. On tangerine, valleys expose you to ship fire, as well as having wide open entry points, allowing a very easy defence stratagy to be used. Crossing valley should be done with artilllery support when possible or an overwhealming force, preferably fitted to destroy large amounts of tanks.