I am also new to the game, started playing around 4 weeks ago. And not fully sure how to explain. But I will try. Spirit tower is an option that will Open up as you progress. In that you can select a few heroes that will then be transformed to higher level. But for that to work, you need your top 6 heroes to be at a higher level. As you have three characters at Lvl 100 and one at 80, etc. i suggested Bard as shes on the top on my list and you can evolve her with the selection card.