Peasants

[steelix]

Its in their nature to run away from attacked. How do I make them retaliate instead of run away? Is there an option for this, or do I have to make another unit take on their appearance...

 

[Ezekiel12]

Its in the nature of the unit classification 'worker'. If you remove it, the most noticeable thing is that they won't show as idle workers anymore.

 

[MN Lahmar]

You can Try Changing Unit Classification in stats or go with your idea.
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Ok this is not an answer but i wanted to say this. Back in the days, we were playing warcraft and an orc raider attacked my peasant and my peasant ran away but when the raider trapped him he attacked him.

 

[Dr Super Good]

If necessary you could make a trigger that in response to them being attacked and not having an order it makes them attack the attacking unit. It is important that they do not have current orders as otherwise you could exploit the mechanic to Ping-Pong them or make micro managing them harder (you cannot order them to run away).