For what its worth, in antiquity there really wasn't an attacker or defender per se (except in sieged obviously). Often if one side wanted to fight but the other stayed in camp, there would be no battle. So a battle only occurred when both sides had a good reason for wanting to fight (confidence in victory, morale or supply issues, time constraints, etc).

Therefore, for a truly accurate battle, both fighters should advance and meet in the middle of the field, as forces did in antiquity. The way RTW battles are fought, with one side waiting patiently on one side while the enemy threw themselves at their line, was rather uncommon. Probably a product of modern conceptions of ancient warfare with rows of steely-eyed Romans standing motionless while wild hordes of barbarians toss themselves feebly against the Roman shields.