Regarding a retreat-result on the dice, the rules say:
"If the target was not eliminated when taking hits,…"
Does this mean, an unit must take at least one hit to be forced to retreat when the attacker rolls a flag symbol?
Or does taking hits even mean that the defender takes 0 hits?
Or what when the defender is hit by one die result, but can avoid to lose a figure due to an effect (toughness i.e.).
I do not find this rule very clear.
No, retreating and taking hits are intially treated separately (though "retreats" CAN become additional hits under certain circumstances).
Step 5 in the Combat Sequence is "Determine Attack Dice," followed by Step 6 "Roll Attack Dice" which is then followed by Step 7 "Resolve Attack."
Step 7 Resolve Attack has four sub-steps:
a. Causing Hits with Colored Shield symbols
b. Causing Hits with Valor symbols
c. Taking Hits
d. Causing Retreats with Morale symbols
So, no matter what combination of die faces one rolls, they are applied in a specific order. A unit that is attacked takes hits first, then suffers retreats if Morale results are rolled against them.
A defender could "take zero hits" and still be forcedd to retreat, if I understand your question correctly, if no hits were caused by Colored Shield and Valor results rolled, but a Morale result was rolled, and the unit being attacked was NOT "Stalwart."
Yes, the Keyword: Toughness 1 allows a unit to ignore the first hit applied to the unit from an attack - but this is also a separate issue from the main subject above I think.
I had some trouble understanding this section of the rules myself, stick with it though, it's a freakin' awesome game.