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.
Swedish Translation
Varje torn har sin egen, unika färdighetsträd. Ju mer de är engagerade i strid, desto snabbare kommer de att få erfarenhet. När de når en ny nivå får de en färdighetspoäng som kan tilldelas i deras träd.
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.