Its also a little buggy in that you can't build anything on them, you have to chop the jungle, then you can build the farm/etc, hmm should check if thats been added to the bug post yet.
The reason, if you're interested, is that many improvements require the improved tile to produce at least one food. Marsh gives one food; jungle removes one food, so jungle marsh prevents any improvement that requires food.
It can work in reverse too. Cottages requires one food to build, plains hills don't produce food. However, if you somehow manage to get ancient forest on the tile, you can replace it with a cottage.
It doesn't make a lot of sense in vanilla either. My guess is that it's a remnant from a cut feature.
Might have just been a hack to prevent people from building things(farms, cottages) on ice/desert in vanilla?
Nah, it would definitly have just been easier for them to block farms on desert/snow. They added it because the thought is that you can farm an area that is "infertile". Defintly easy to remove this requirement from farms, but their definition made more sense and was more flexible (if people added/removed terrains, etc).
Actually seems LESS flexible, since if I am understanding right it is impossible to create unfarmable terrain except by giving it 0 food value. For example, I'd like to make Hell terrain unfarmable, just to see what it is like balance wise. However, there's no way I can do this without removing the food value from hell terrain, which is obviously not ideal.
Nah, it would definitly have just been easier for them to block farms on desert/snow. They added it because the thought is that you can farm an area that is "infertile". Defintly easy to remove this requirement from farms, but their definition made more sense and was more flexible (if people added/removed terrains, etc).
Actually seems LESS flexible, since if I am understanding right it is impossible to create unfarmable terrain except by giving it 0 food value. For example, I'd like to make Hell terrain unfarmable, just to see what it is like balance wise. However, there's no way I can do this without removing the food value from hell terrain, which is obviously not ideal.
This is such a weird argument to me. You have a variety of things you can place requirements on. You can place them directly between improvements and terrain if you like. Or you can have something else qualify it, such as the food yield.
Using the food yield gives you different options than a terrain requirement. Not better or worse, just different. For example since farms are based on a food requirement on the plot the Illians can farm snow tiles, since they get food from snow tiles.
Likewise a direct terrain mapping has benefits in that you can ignore the terrain yields, all that it needs is the terrain requirement.
Either way you prefer is available through xml. So its all good stuff. Just like units can require a tech, a building, a religion, etc etc. You wouldnt argue that it was bad design on Firaxis's part to have units be able to require religions. You could argue that you dont like that a certain unit requires a religion, but then its avilable to you to customize. Its the same arggument for farms.