TymC
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Well, the last two definitely need their stats tweaked, their armies expanded, and maybe some artifacts added (and I don't know how this would be implemented, but it's common on other maps) so that they always attack the city. Otherwise, they'll just stand around or run away from the hero, and the whole point of the map will be missed. Also, when making it more difficult, you could add a couple of guarded mage huts with demonstrations of interesting remote locations. And having an artifact shop near the castle, when there's plenty of gold, seems like overkill to me. I'm not a fan of hardcore difficulty; I play some maps even on Veteran. But to enjoy the game, you need at least some challenges to overcome heroically.
I added an army to them when I removed the monastery; the mage huts are an interesting idea. I'll do it as best I can. I tried to make him aggressive from the start by setting the highest priority, but apparently, even then, he doesn't go on a suicide mission. I'll probably also move the artifact shop further away.