If the enemy attacked you, it means that your ambush must have failed I guess - otherwise you'd have been able to set up with the enemy arranged in a column (unless I'm totally misunderstanding you!)

I had a similar thing happen to me - my army was set for an ambush, but the Carthaginian army attacked me - it said my attempt at an ambush failed, so I just had to do a regular attack instead... I guess my commander forget to tell everyone to walk on tip-toes...