Except those last few levels, virtually impossible on impossible mode with the free-tier heroes:-(. I tend to like the original and Frontiers much better, though ironically Origins was one of the most "balanced" of them all, and that was neat to experience as well:-).

e.g. archer towers should shoot singly, fast, and at high range, while barracks hold the line and are focused more on defense, right? Nope, the former do AoE magic bombs instead, while the latter shoots arrows into the sky. And artillery should focus on offense and hit AoE in as wide an area as possible, right? Nope, there are tanky bears and the shots hit only a narrow space. I'm cherry-picking these examples, but you get the idea - every tower does stuff that overlaps with what you would think other tower types might do instead, which makes for some fun thinking to deal with the foes.:-D