The games gets a lot harder as it progresses. As usual, normal is a nice and fun difficulty to start and get acquainted with the game and the map. However, on hard things change. You will see very strong and overpowering enemies pretty often on normal, even with a warrior. On hard overpowering enemies are pretty common. I'm yet to play the paladin on hard, and then will move on to impossible, but right now, in the late mid-game and nearing the final part of the game, I barely see an enemy that is not overpowering compared to my army. That's not a big cause for concern, of course - with proper units, spells and tactics you can easily beat the crap out of impossible enemies without much hassle.
So the bottom line is that impossible will now be a real challenge, not like in KBTL, where you could easily move from normal (just 1 play-through to get acquainted with the game) directly to impossible.
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