English (Source)
Each towers has their own, unique skill tree. The more engaged in combat they are, the quicker they will gain experience. When they level up they gain a skill point which can be allocated in their tree.
Chinese Translation
每个塔都有自己独特的技能树。他们参与战斗越多，获得经验的速度就越快。当他们升级时，会获得一个技能点，可以分配到他们的技能树中。