It is a military axiom not to advance uphill against the enemy, nor to oppose him when he comes downhill.
Being above the enemy gives the advantage of potential energy. You can use gravity to move things down on them, from rolling rocks to projected missiles. You stand higher than them and can swing down on top of them.
There is also a subtle motivational psychology where height is a very common metaphor for superiority. People who are higher feel superior whilst people who are lower feel inferior. With this effect, they may well be motivated and demotivated in a battle.
This can be seen in animals who prefer to stand higher. In humans also, height is a symbol of status. There are more taller people in senior business roles. Kings sit on daises.