Problem is that the in game tools are somewhat limited, in that digging out round sections, or constructing round towers is a PITA to do due to how many steps it requires. Building a simple square structure probably requires an order of magnitude less steps than a round one does, even if the round structure is smaller than the square one.
I think I'll colonize a cavern layer with my next fort, though I'll continue to tinker with this one, perhaps colonize hell just for the sake of doing so, and then once DF2012 is stable I'll invade a layer of the caverns, wall off the entire caverns so its safe, level the terrain because I like my forts to be all on the same level, more or less, for convenient viewing, and then leave the natural stone pillars as towers.
Thinking something inspired by something like this:
http://www.youtube.com/watch?v=w1kAwnGoqcMOn a side note, what inspired Minedances:
http://www.youtube.com/watch?v=3Eh22nIFZQUI used freestanding towers in the middle and didn't carve much into the walls, but its the same sort of city sunken into an open pit or stripmine type idea. Another thing I could try would be to build a city much more like that. Probably a smaller pit in the middle, maybe just 30x30, filled with magma of course, with the city carved into the stone walls. Up top there would be fortifications similar to the ones in Minedances, extending all the way to the edge of the map, with a gatehouse garrisoned entirely by soldiers and not using any weapon traps.
Downside to this design is that turning temperature on would be murder for my FPS. Upside is that embarking on a snowy biome would provide an interesting mix of snow, ice, and magma. I'd also need to get better at designating round shapes for the central volcano core.
I think colonizing caverns might be the better option overall.