Every victorious battle will earn your units XP (Experience Points). Once units gain enough XP they will gain a level. Units trained in a city will see their stats raised when they level up. Every time a Champion levels up they not only get a boost to their stats but can also select a new trait to learn from their Ability Tree. Traits provide a variety of effects to improve your units base stats or give them new abilities to use in battle. Each Champion has their own unique Ability Tree, so be sure to experiment with the wide variety of traits available to each.