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.
Japanese Translation
各タワーには固有のスキルツリーがあります。戦闘に多く参加するほど経験値の獲得が早くなります。レベルアップするとスキルポイントを得て、ツリーに割り振れます。