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.
English Translation
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.