I've answered this before in this thread (by me, /u/superplaner, I see you AutoModerator). Feel free to have a look and if you have any follow-up questions or want specific source material I'll be happy to help. A very brief TL;DR is that the defender usually gets to pick the battlefield by simply moving to a place they find suitable and waiting for the attackers to come to them. This is what happened at famous battles such as Agincourt, Poitiers and Formigny. The exception is if the attacker catches the defender unaware, which has happened although it is fairly rare. In this case they can either move to an ambush position or attack immediately.