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Not on every map. There are random numbers, so you can set the probability of losses to, say, 3% or so. And it should be annoying.
The terrain becomes challenging.
If randomness is planned, the player has a way out in the form of save/load. :)
You can place PE (presumably, Permanent Enemies) not so densely, or near resource sources, or cities - depending on the map.
"Hostility" can be implemented by, for example, increasing the number of neutrals.
Or make the passages paid. Or... there are many options.
Ваяю кампанию под HoMM4, обновленный мод Equilibris ver. 3.55 <заморожено>