With each passing second youll receive a small amount of cereal that you can either use to recruit more soldiers or to increase your cereal production. The key here is to create a balance between increasing your cereal production and recruiting the soldiers thatll defend your castle. Using this strategy is the only way to get rid of your enemys castle.
Kingdom Wars has over 400 different levels where you can face the rival army. Luckily, you have more than a hundred different units that you can upgrade little by little using the earnings you get after each battle. You can even upgrade the castles barriers.