Holding specific terrain practical in real wars?

[max_torch]

In wesnoth it is difficult to defeat an opponent who is holding the right terrain. Even worse if the terrain you have to stand on to attack them is a difficult terrain. Does this have any realistic inspiration from real-life fighting? I suppose it's like "holding the high ground"?

[Vyncyn]

Sure it does.
Fighting from uphill gives advantages, beacause you can see further and the enemy gets exhausted easier. Hill terrain can also be used to hide a part of your army.
Forest can serve as ambush aswell as source of food and wood.
If you hold the coastline of a river the enemy most likely can't cross it. The borders of many european countries are at rivers or other natural obstacles because of this.
Castles are often placed near cliffs or rivers because it's easier to defend there. And the enemy can't ignore those castles as they pose a constant threat as long as they are occupied, since you don't want to have enemy forces behind your lines.
Cities are keypoints to trading and supply routes. They also hold a large amount of resources, the walls are easy to defend and they give your army a place to rest. As long as you can hold them you can strip the enemy of those advantages.

In a 1v1 melee terrain would matter less, but if the enemy has to fight from lowground or in water he would still be at a serious disadvantage.

[Sudipta]

Its also very important from the movement perspective as well as the battle perspective pointed out above. Its easier and faster to move an army over flat terrain and when there are good roads. Speed and tactics are one of the deciding factors of any war. Take a look at Napoleon's conquests or the German army's Blitzkrieg during the World Wars.
Its very difficult to move an army over frozen terrain and in freezing conditions which is one of the main reasons for Russia and Kievan Rus's victories in historical wars, since the invaders aren't used to the cold and the rough terrain and take massive attrition damage.
Rivers and seas allow for fast transport of troops and supplies so controlling them is very important.
It's difficult to defeat opponents who are entrenched or defending in caves or tunnels, because they have underground networks allowing for secret movement, and also its difficult to estimate their numbers.
For the forest case, a great example is the Vietnam War, the dense forests of Vietnam is one of the main reasons why the Americans lost despite their huge technological and air superiority.
Narrow choke points allow a smaller army to hold their ground more effectively against a numerically larger force, the best example i can think of is the historical Persian invasions of Greece.
So yeah, terrain matters both in game and in life.