Very fun game. I want more content, maybe another 10 levels. As for improvements:
I don't think you should hide the perk tree, especially considering there is zero penalty to refunding perks.
Your power scales up too quickly. Early perks are way too strong.
I was able to easily bull-doze levels without considering the enemy's strategy at all. This strategy is straightforward/obvious and beats everything, from start of the game with no prestige until the end:
Grab enchanter for the back slot and upgrade heal/health. Throw wayfarer in a middle slot for free damage. Put a strong hero in first and dump all of your money in. Eventually, put sharpshooter in second and dump money in it as its power grows to quadruple thanks to its crit growth and its ability scaling with crit.
You probably want a buff/debuff hero that is less end game that can interact with the rest of the heroes in a more interesting way.
Villager is neat, obviously strong, but I only used it for the last level. Similarly, items felt a bit win-more..nice to grab for the price, but totally unnecessary. Probably could have ignored both easily.