I don't need to go find other examples (although there are many). In each of those instances I already mentioned, one side intentionally chose to fight in terrain that was not open. It was the exact opposite and that's why they chose it. The terrain gave them a tactical advantage or undermined an advantage their enemy had.
Sorry, but this is just not true.
The fighting in all three cases took place in flat, open terrain.
They were fighting in the open terrain.
Trasimene:
http://de.wikipedia.org/w/index.php...lachtgelände.jpg&filetimestamp=20080508095250
Thermopylae:
http://en.wikipedia.org/wiki/File:Thermopylae_ancient_coastline_large.jpg
Teutoburg:
http://de.wikipedia.org/w/index.php?title=Datei:Varus01.jpg&filetimestamp=20090324132811
In the case of Varus' battle at Teutoburg Forest (which actually at that time may not have been as open as depicted), don't forget that this was not only an ambush, but that the Romans didn't expect any battle or even fighting at all, as Arminius was supposed to be an ally.
I will agree that the majority of large battles in the ancient world were fought in relatively open terrain (of course, most ancient civilizations existed in the open flood plains of major rivers so that was kind of bound to be the case), but that does not undermine the fact that open terrain carries disadvantages for the army fighting there. That is exactly the effect modeled in Civ V. If I one places his or her army on a hill and the enemy is before them in open and flat terrain (remember hills can be "open" as well), the army on the hill has the advantage in the defense. It may even have an advantage in the offense given it's superior position. I don't think that's really debatable.
Once again, this is not (completely) true.
1) Hills
I will admit that an attacker charging downwards from a hill will have an additional advantage due to the higher speed he gains by going downhill.
But this is only true for a rather short distance, as neither men nor horses will run over long distances without getting exhausted. Furthermore, the sides of the hill aren't allowed to be too steep nor shall there be too many obstacles (creeks, bushes, ground waves, whatever).
This limits the area considerably to only some hundreds of meters.
This distance on the other hand enables the defender to prepare (individually) for the charging, even more if being aware of the enemy's presence.
In that case, it was typical for the ones in the lower terrain to build something like light barricades, dig some trenches and what not more.
2) Forests
In earlier times (up to and including the beginning of the 20th century) you would avoid to fight in forests as you would lose control over your units.
It may have happened that parts of your units would have to fight in the forests, but only in the context of the main battle taking place in the open.
In both cases, latest after the opponent has entered the same terrain, the advantage of the defender due to terrain is gone.
Actually, forest give you a defensive advantage when they have to be entered by the attacker, in other words, while his forces are still outside of the forest.
After that, it comes down to fighting man against man, with the result being open.
Similar it is with fighting in hilly regions. As soon as the attacker has entered this region, the advantage lies with whom has the higher elevation, which typically can change every hundred meters.
The display of the terrain's influence in Civ games is completely distorted.
If we were to go into detail about the scale of the maps, it would become even worse.
In total, the influence of hexes/tiles in Civ games on combat is completely unrealistic, unplausible and misleading.