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
每座防御塔都有自己独特的技能树。它们参与战斗越多，获得经验就越快。升级时会获得一个技能点，可分配在其技能树上。