It was decided that the location for the final battle was would be a valley that was slightly flatter.
Of course this location was not the most ideal place for the final battle. However, this was already the best spot that Leon could find that was located between the witcher-knights and the Mages' base.
Due to the continuous harassment by the vampires and the bug swarm, the witcher-knights had to tighten their formation to avoid exposing any openings to the enemy. This caused their marching speed to be as slow down to a tortoise's speed. But even so, they still arrived at the decided battlefield at midday.
This was a place with sparse vegetation and less mountainous terrain. On both sides were rocky hills and flowing streams. Mountain ranges and steep cliffs were everywhere to be seen. If the witcher-knights wanted to continue north, this was the best location for an army their size.