Boris K
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In my opinion, the enemy should appear immediately after capturing all dwellings. It seems there's also a time limit for this, and an additional objective to capture the gold mines. Maybe the time has expired?
I wouldn't recommend playing through the campaigns on Recruit difficulty – it's the most buggy difficulty level.
In my opinion, the campaigns in Might and Magic: Heroes are the easiest. I don't remember any difficult missions. I completed them very easily. That's why I don't remember them very well.