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In every Ambush; legions, forts and citadels are removed without rolling(to absorb the loss), and the defender does NOT counterattack.To expand on this a bit and answer the specific questions: It is one die roll for the region where Caesar is located. Not one roll per unit. That one die roll is used to negate all effects of the ambush in the region (die roll for losses, retreat/counterattack)
EXCEPT, if Caesar is present: The Romans then roll to see if they counterattack and if they may roll to absorb losses.
-If the Romans roll 4-6 they may do so.
-If battling the Belgic, the Romans need to roll 5-6 to do so.
The Battle Procedure player aid is a handy reminder for this.
Basically, if the region with Caesar is selected for an ambush/battle the Roman player may roll a die and if it's within the ranges woxoya stated then that region is effectively only selected for battle without ambush.