Maybe you do this already but I'm guessing you don't from what I've seen so far.  I think it would be a good idea to make resources more dependant on terrain.  Iron and gold are found more on hills and near mountains, food is more common on the plains and old growth forests are usually found near or in forests.  It would give terrain more meaning, would feel more realistic and would help the player when looking for a particular resource.  If I'm desperately in need of iron than I can follow the mountain range and have a better chance of finding what I'm looking for.