I mentioned this in another thread, but, it would seem to apply here aswell.
I live in Florida, USA.
The highest point in my state is 353 ft above sea level.
In Civ terms my state is basically made up of grasslands, plains, swamps (which could be jungle), and perhaps flood plains.
So, how high does the land have to be to beciome a hill?
Normally, the land would have to be high enough to be of strategic value for someone to "take the high ground" as has done in many battles.
Maybe, start at 1000 feet?
But, then go up to where?
Much of the Appalachian mountains would qualify for this. They average around 3000 ft, with a max of 6684 ft.
http://en.wikipedia.org/wiki/Appalachian_Mountains
So, it should be a mix of hills and a higher terrain.
The rocky mountains mostly start at this higher terrain and go even higher to full Civ mountain status.
So, my point is Civ needs to add this middle terrain, that is higher than the lowly hills, but lower that the highest impassable peaks.
IMO, Denver, CO isn't a hill, nor a Everest type peak. It is some thing inbetween.
A Civ terrain could include:
Hills: 1000 ft - 2999 ft (mine able, and passable) +25% Terrain defense
Mountains: 3000 ft - 7999 ft (mine able and passable with roads technology) +50% Terrain defense, unless the terrain below is hills, the just 25% more.
Peaks: 8000+ ft (not mine able or passable)
The defense matters from where the attacker is attacking from. The more level the ground, the less tactical the advantage is. Air units would ignore ground defense.
Anyway, you get the idea about different types of terrains.
Hills, Mountains, and Peaks.
Oh, and I looked up Machu Picchu (Quechua: Machu Picchu) "Old Mountain", pronounced [ˈmɑ.t͡ʃu ˈpix.t͡ʃu]) is a pre-Columbian Inca site located 2,430 metres
(7,970 ft) above sea level.
http://en.wikipedia.org/wiki/Machu_Picchu
It would fit my example of a Mountain terrain.