I design things on aesthetic, on metaphor, on symbolism, on what will, perhaps, be interesting to destroy. I care not for sanitation, the environment, or common sense. I am reminded of a painting by the artist Greg Bridges, who operates a coffee shop in my neighbourhood. The inside, filled with old odds and ends he has picked up over the years, has his various bits of his art in it, including a painting of a city built on the edge of a cliff. Hanggliders, or kites, or balloons, move around in the background--I cannot remember exactly--but the city sits right on the edge of a cliff, and has various bad natured elevators and sudden drops, while a dirty plain leads off panel. It's a cool painting, and when I am in the shop, I look at it and let my mind turn over all the ways that, while a visually distinctive and neat piece of work, it's an amazingly poorly designed city, a city in which children fall to their deaths every day, where suicides are high, employment low, and invading armies lay siege and then simply starve everyone out. It looks cool, but it's hugely impractical in terms of design and sustainability, and if you followed the lay of the land suggested by the painting, could have easily been built in a much saner place.
Though, y'know, it would not have looked as cool.
There are lots of examples that I am sure you can think of, from books both large and small. They are cities built in largely impractical areas with largely impossible designs--either by their site or by their material--in which, if you stopped and thought about it, must result in at least hundreds of accidental deaths each year. A lot seem to be built on the top of mountains, though I am sure there's enough on volcanoes, ocean edges, and other poorly selected locations that a rather long and terrifying list can be created. The Impractical Cities of Fantasy, you could call it. But it doesn't really matter--in the same way that you don't stress the poor dental hygiene, the defined lack of factories to produce toilet paper, the sewage systems, and likely plagues that break out every few years from the combination thereof, you don't really stress the Improbably Design of Fantasy Cities (that's a better title, surely?) and so long as they work within the text and do not break the world (literature works on metaphor and symbolism, remember) you're all good.
Which is good, because that's the balance I run. I would be wasted designing real cities, planning urban sprawl and environmental existence, on assuring native life still has places to exist next to huge towers. I see the side of a cliff and I say, "My giant stone castle is going to sit right there," and I don't stress the weight of that on the overhanging cliff. To me, the fantasy is not just in the immortals, the gods, the magic, the fighting and the like, but it's in the design of the world, in the ability to use your literary designs in big, fabulous ways. A fantasy novel is a big, wide screen experience--by its very nature it is not small and isolated in tiny rooms--and I reckon its best when you go with that in design especially. I like giant stone canals. I like huge bridges to link islands together. I like the complete and utter madness that lets me create something wild and insane and which, in the end, would be shaken off by any government as ridiculous. Well, until you stop and think about San Pedro Prison, at any rate.