Battles Were Fought by Mutual Consent
Most battles were fought on relatively flat and open plains, with terrain used (if at all) only to shield one’s flanks. This is partly because of the unwieldiness of the formations involved, and partly because the enemy could always choose not to attack a strong defensive position, so battles tended to occur mostly by mutual consent when both sides were willing to fight in the open. On the battlefield, defensive features like river lines could be a double-edged sword, as the enemy could focus his attack wherever he chose, and you would have to attack across the river yourself to exploit his weakness elsewhere. Adopting too cautious and defensive an approach could also shift the psychological balance between the armies, and so prove counter-productive in terms of overall combat effectiveness. Without this moral pressure to come down onto the plain or move away from the safety of one’s camp and take the risk of engaging on equal terms, many confrontations would have ended in indecisive stand-offs, given the enormously high stakes involved when the losing army was likely to be completely shattered and have its troops massacred in the pursuit.
— Philip Sabin, Lost Battles, pp. 222-23