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. If there's only one option, the tower will automatically level up.
Chinese Translation
每座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，可分配到技能树中。如果只有一个选项，塔会自动升级。