Cities as magic centers?
In some civ-like games, cities have the options to specialize as centers of production, or learning, or agriculture, or religion. In this upcoming game I was wondering if cities could also have the option of specializing in magic. I think this option is most interesting when this sort of specialization comes with a cost (you can't be good at everything). For instance- a city located in the mountains to incearse its mining production may not be great a producing food.
I see magic-centered cities (maybe for the specialization of just one type of magic) as having a problem with population. If you find a spot on the map that would be great for a "magic city" (like a holy mountain, or a site with a crystal, or some other source of power from the first age (or whatever))-it is probably not a spot most of your population wants to live. I would imagine that most non-magical folks would treat magic sites like some radioactive dump-with a high degree of suspicion. Therefore I suggest that in a normal city-higher level magical buildings will cause unrest or have some other detrimental effect.
Gameplay: You could still get along just fine without this type of specialization-it just adds some options. Magic cities could be founded to terrain that would not normally support large cities. Since these cities will never get very big, there will not be enough man-power for normal defence. In dire situations, you can bring an army in to protect it, but this will dispell any illusions(no longer hidden) and maybe some other magical protections, and non-magical troops (if not fighting) will slowly leave the area (their numbers will go down).
But, being a magical site, special spells can be researched and developed to magically protect these centers. for instance
low level spells- +1 or +2 protection for defence
mid level spells-site is hidden from view for non-magical unit. non-magical cannot enter site, construction of magical walls or domes
high level spells- demon or similar monsters guarding the site.
Other considerations: bonuses--wizards or magical beings are produced faster, spell-learning rate is higher than in normal cities, spells cast there could be a bit more powerful, no unrest caused by magical buildings.
This allows for two very different communities with the potenitally interesting interactions. Allow for the "hidden city" thing to happen which I personally like. Allow for more choices on where to put your resources. Allow for more civic choices (many some will allow for better or worse communication between the two city types). Allow for using more of the map (not just the fertile plains). Allow for special spells for early protect of these special cities. Allow for more interesting stories within the game -the development of "hallowed sites" and their potential destruction by other forces.