Because the mountain top is the commanding height and has a wide field of vision, it is not only conducive to detecting the enemy's movements, but also an excellent sniper position at an important pass. Moreover, it is easy to defend from above and difficult to attack from below. Of course, not all hills are good for defense. Therefore, the hills generally chosen are the only way for the enemy, and it is best to have a small defensive surface or a steep hill that is conducive to defense and not conducive to attack.