I thought this was a nice little game to play. I'm oddly addicted to passive games even though I never really see the point in them. :P I guess it's the medals/achievements that keep you coming back and wanting to play them.
My first run through I basically played with no strategy. I bought heroes as soon as they appeared, no matter how much they cost and I just purchased upgrades randomly. The second time through I played with some strategy and if anything there was maybe only a 5 minute difference between gameplays with barely any difficulty change. So the game is kind of not challenging no matter what you do. I never used the pause feature on either run through.
I wish the map would have been larger. It may have added an additional challenge if there were several more pages to the map with more adventures as people may have gotten distracted enough to forget to buy potions. I see that you say each character has different effects, but is that obvious anywhere on the screen? Is that information someone can even figure out and use as a strategy or is it just randomized?
The sound, music, and art work was all really fitting for the gameplay. Overall, I enjoyed it, but would have liked to see a little bit more of a challenge and a bit larger map/mission selection.