In the Middle Ages, to hold the high ground was certainly advantageous, for several reasons. First and foremost, regions of local elevation would be often be sought out prior to an instance of military action as it allowed sufficient view of the surrounding environment, and gave an opportunity to observe an enemy movement. Occupying high ground also offers several natural defensive advantages. Armies on a plane of higher elevation are able to achieve longer range with missile weaponry, and are similarly harder to hit with long range attacks without the attacking force first entering the range of the defenders. It would have been more difficult for medieval armies to have transported heavy equipment and other materials up a slope.

In terms of actual combat, an individual soldier would have been in a stronger position when holding high ground. It is more tiring to move up a slope than it is to move down one. A soldier attacking uphill would have to expend more energy on constantly looking up at their foe, which could cause visibility issues when wearing protective headgear such as a helmet, which often impaired the field of vision. A soldier standing higher than another would have to defend his lower body and legs, which could be achieved by a crouching position, while the individual standing below him would have to expend significant energy holding up his weapon or shield in order to protect his head and upper body from blows from above.

The basic defensive advantages of high ground are most evident in medieval defensive fortifications. As a general rule, castles and forts in Medieval Europe will built on local rises in elevation, for the advantages mentioned above, as well as taking away the possibility for the attacker to easily fire down missiles onto those within the fortification. In terms of Medieval strategy in battle, there are numerous accounts of military forces occupying high ground for strategic purposes. One such example would be the Battle of Fontenoy (841 CE), where Charles the Bald maneuvered his forces to stand on a hill shortly before the battle commenced.

It should be noted that the natural advantages mentioned are defensive ones. The advantages are somewhat neutered if the army was the attacking force, which often necessitates advancement and movement, which can be constrained if a force was determined to hold a particular area. The lack of movement in such a strategy could allow for the defending force to be encircled and besieged, an act that is more easily accomplished in elevated environments that do not have a full circumference of escape routes, such as a cliff or mountainside.

As to the relative effectiveness of high ground, I cannot attest to what sort of percentage the defenders holding the high ground would win, as asked in the post. What I can say is that holding the high ground is especially advantageous when adopting a defensive strategy, but it was certainly not foolproof. While environmental factors often play a tremendous role in Medieval battles (i.e. the Battle of Agincourt), plenty of other factors contributed to the likelihood of tactical success, such as force size, equipment, troop type, the mental/physical health of the troops, and military/strategic prowess of the commanders.

An example of losing an engagement while holding the high ground would be the Battle of Hastings (1066 CE). Harold Godwinson's forces occupied the high ground, but William of Normandy used the strategy of feigned retreats to allow Godwinson's troops to break rank and pursue fleeing Norman troops ( or at least destabilize the formation of the shield-wall). The subsequent counterattack by Norman troops led to a decisive defeat of Godwinson's forces.

Hopefully I was able to somewhat answer your question.