Another Roman custom, that I thought took place on the Campus Martius but I'm not seeing it, involved the formal declaration of war. In Ye Olde Days when Rome was but a city-state, actual ambassadors (of sorts) would present themselves as official Adversary. Later, when the frontiers made such cozy arrangements impractical, they were simulated for the occasion.  It might seem silly, but this was letting the gods know Rome was at war and against whom.