raystuttgart
Civ4Col Modder
I know that mountains and hills were not in that list before, ...
As @Nightinggale explained, they are not "Terrain Types" and they are also not "Terrain Features".
They are "Plot Types" which is technically a completely different concept with its own implementation rules, hardly visible in XML - only by a few modifiers.
... but would it not make sense to add their description to base terrain, too?
No, because you can not define the outcome of Yields just by e.g. "Mountain" or "Hill" itself.
You always need to know the combination of "Terrain Type" + "Plot Type".
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To simplify it for Players:
- Base Terrain (in Colopedia) = Terrain Types
- Peak, Hill, Flatland, Water = Plot Types
The Base Terrains more or less tell this:
For Land Terrains:
- Base Terrain + "Flatland" (which is default for "Land Terrains")
- Base Terrain + "Water" (which is default for "Water Terrains")
What the Base Terrains simply do not tell is this here:
For Land Terrains:
- Base Terrain + "Peak" --> However it can be computed and thus also be displayed
- Base Terrain + "Hills" --> However it can be computed and thus also be displayed
So the question is not if we can display it.
The question is only how.

One idea would be to have in Colopedia -> 2 new "Main Entry" categories
- ...
- Base Terrains (old)
- Base Terrains + Hill (new)
- Base Terrains + Peak (new)
- ...
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@Nightinggale , @devolution , @f1rpo
Do you guys have a better idea?
Or have you already seen somebody implement visualization of "Peak" / "Hill" in Pedia?