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:

• 1. Click on the valley, then click ‘Enter’. Once inside the valley screen, click ‘Recall’ next to the army to return that army to the city from which it was originally sent out.
• 2. Click on the valley, then click ‘Reinforce’. Once inside the March screen, select the armies that you want to send there to defend it, then click OK to confirm. Note that due to a bug in the game, it is not possible to view the type and number of troops that have sent later on. Therefore, it is advisable to archive the “Troops Arrived” message in Reports > Army that you will receive when the reinforcements have arrived as a memo for later reference.

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.