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.
Swedish Translation
Varje torn har sitt eget, unikt färdighetsträd. Ju mer de deltar i strid, desto snabbare får de erfarenhet. När de går upp i nivå får de en färdighetspoäng som kan fördelas i trädet.