When you are conquering a valley, be aware that the higher the level, the more troops it will have to protect it. It is advisable to scout it out before you are going to attack it.
Usually, 30 archers for a level 1 valley, 70 for level 2, and 100 for level 3 will conquer the valley with no troop losses. However, if any troops are sent which aren’t archers they may be hurt, and the archers may have casualties if less then this amount is sent. (Taken from Medal Farming )
Once a valley is conquered, it is under your control and you will receive the associated resource production benefit (see above) while it remains under your control. It is possible to recall your main campaign army (which is led by one of your heroes) and to send a more modest reinforcement (without a hero) to defend your land after that, if your level of Rally Spot allows it. To do this, follow these steps:
Troops reinforcing a valley eat more food then they would at your city, thus many elect to have no army protecting thei valley’s especialy, if the valley is low level and thus easily replaced.