English (Source)
Enemy buildings can be destroyed by building towers in the blue spots next to the building. After each wave, each tower deals 1 damage to the building, which will give you rewards when it's destroyed.
Chinese Translation
可在建筑旁的蓝色位置建塔摧毁敌方建筑。每一波结束后，每座塔对该建筑造成1点伤害，摧毁后可获得奖励。